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Announcing the .NET Framework 4.8

The .NET Framework 4.8 includes an updated toolset as well as improvements in several areas:

  • [Runtime] JIT and NGEN Improvements
  • [BCL] Updated ZLib
  • [BCL] Reducing FIPS Impact on Cryptography
  • [WinForms] Accessibility Enhancements
  • [WCF] Service Behavior Enhancements
  • [WPF] High DPI Enhancements, UIAutomation Improvements

You can see the complete list of improvements in the .NET Framework 4.8 release notes. Reference sources have also been updated for .NET 4.8.

Supported Windows Versions

Windows Client versions: Windows 10 version 1903, Windows 10 version 1809, Windows 10 version 1803, Windows 10 version 1709, Windows 10 version 1703, Windows 10 version 1607, Windows 8.1, Windows 7 SP1 Windows Server versions: Windows Server 2019, Windows Server version 1803, Windows Server 2016, Windows Server 2012, Windows Server 2012 R2, Windows Server 2008 R2 SP1

New Features in .NET Framework 4.8

Runtime – JIT improvements

The JIT in .NET 4.8 is based on .NET Core 2.1. All bug fixes and many code generation-based performance optimizations from .NET Core 2.1 are now available in the .NET Framework.

Runtime – NGEN improvements

NGEN images in the .NET Framework no longer contain writable & executable sections. This reduces the surface area available to attacks that attempt to execute arbitrary code by modifying memory that will be executed.

While there will still be writable & executable data in memory at runtime, this change removes those mapped from NGEN images, allowing them to run in restricted environments that don’t permit executable/writable sections in images.

Runtime – Antimalware Scanning for All Assemblies

In previous versions of .NET Framework, Windows Defender or third-party antimalware software would automatically scan all assemblies loaded from disk for malware. However, assemblies loaded from elsewhere, such as by using Assembly.Load(byte[]), would not be scanned and could potentially carry viruses undetected.

.NET Framework 4.8 on Windows 10 triggers scans for those assemblies by Windows Defender and many other antimalware solutions that implement the Antimalware Scan Interface. We expect that this will make it harder for malware to disguise itself in .NET programs.

BCL – Updated ZLib

Starting with .NET Framework 4.5 we used the native version of ZLib (a native external compression library used for data compression) from http://zlib.net in clrcompression.dll in order to provide an implementation for the deflate algorithm. In .NET Framework 4.8 we updated clrcompression.dll to use version 1.2.11 which includes several key improvements and fixes.

BCL – Reducing FIPS Impact on Cryptography

.NET Framework 2.0+ have cryptographic provider classes such as SHA256Managed, which throw a CryptographicException when the system cryptographic libraries are configured in “FIPS mode”. These exceptions are thrown because the managed versions have not undergone FIPS (Federal Information Processing Standards) 140-2 certification (JIT and NGEN image generation would both invalidate the certificate), unlike the system cryptographic libraries. Few developers have their development machines in “FIPS mode”, which results in these exceptions being raised in production (or on customer systems). The “FIPS mode” setting was also used by .NET Framework to block cryptographic algorithms which were not considered an approved algorithm by the FIPS rules.

For applications built for .NET Framework 4.8, these exceptions will no longer be thrown (by default). Instead, the SHA256Managed class (and the other managed cryptography classes) will redirect the cryptographic operations to a system cryptography library. This policy change effectively removes a potentially confusing difference between developer environments and the production environments in which the code runs and makes native components and managed components operate under the same cryptographic policy.

Applications targeting .NET Framework 4.8 will automatically switch to the newer, relaxed policy and will no longer see exceptions being thrown from MD5Cng, MD5CryptoServiceProvider, RC2CryptoServiceProvider, RIPEMD160Managed, and RijndaelManaged when in “FIPS mode”. Applications which depend on the exceptions from previous versions can return to the previous behavior by setting the AppContext switch “Switch.System.Security.Cryptography.UseLegacyFipsThrow” to “true”.

Windows Forms – Accessibility Enhancements

In .NET Framework 4.8 WinForms is adding three new features to enable developers to write more accessible applications. The features added are intended to make communication of application data to visually impaired users more robust. We’ve added support for ToolTips when a user navigates via the keyboard, we’ve added LiveRegions and Notification Events to many commonly used controls.

To enable these features your application needs to have the following AppContextSwitches enabled in the App.config file:

Windows Forms – UIA LiveRegions Support in Labels and StatusStrips

UIA Live Regions allow application developers to notify screen readers of a text change on a control that is located apart from the location where the user is working. Examples of where this would come in handy could be a StatusStrip that shows a connection status. If the connection is dropped and the Status changes, the developer might want to notify the screen reader of this change. Windows Forms has implemented UIA LiveRegions for both the Label control and the StatusStrip control.

Example use of the LiveRegion in a Label Control:

Narrator will now announce “Ready” Regardless of where the user is interacting with the application. You can also implement your UserControl as a Live region:

Windows Forms – UIA Notification Events

In Windows 10 Fall Creators Update Windows introduced a new method of having an application notify Narrator that content has changed, and Narrator should announce the change. The UIA Notification event provides a way for your app to raise a UIA event which leads to Narrator simply making an announcement based on text you supply with the event, without the need to have a corresponding control in the UI. In some scenarios, this could be a straightforward way to dramatically improve the accessibility of your app. For more information about UIA Notification Events, see this blog post.

An example of where a Notification might come in handy is to notify the progress of some process that may take some time.

An example of raising the Notification event:

Windows Forms – ToolTips on keyboard access

Currently a control tooltip can only be triggered to pop up by moving a mouse pointer into the control. This new feature enables a keyboard user to trigger a control’s tooltip by focusing the control using a Tab key or arrow keys with or without modifier keys. This particular accessibility enhancement requires an additional AppContextSwitch as seen in the following example:

  1. Create a new WinForms application
  2. Add the following XML to the App.config file
  1. Add several buttons and a ToolTip control to the application’s form.
  2. Set tooltips for the buttons.
  3. Run the application and navigate between the buttons using a keyboard:

Windows Forms – DataGridView control accessible hierarchy changes

Currently the accessible hierarchy (UI Automation tree) shows the editing box tree element as a child of currently edited cell but not as a root child element of DataGridView. The hierarchy tree update can be observed using Inspect tool:

WCF – ServiceHealthBehavior

Health endpoints have many benefits and are widely used by orchestration tools to manage the service based on the service health status. Health checks can also be used by monitoring tools to track and alert on the availability and performance of the service, where they serve as early problem indicators.

ServiceHealthBehavior is a WCF service behavior that extends IServiceBehavior. When added to the ServiceDescription.Behaviors collection, it will enable the following:

  • Return service health status with HTTP response codes: One can specify in the query string the HTTP status code for a HTTP/GET health probe request.
  • Publication of service health: Service specific details including service state and throttle counts and capacity are displayed using an HTTP/GET request using the “?health” query string. Knowing and easily having access to the information displayed is important when trouble-shooting a mis-behaving WCF service.

There are two ways to expose the health endpoint and publish WCF service health information: by using code or by using a configuration file.

  1. Enable health endpoint using code
  1. Enable health endpoint using config

Return service health status with HTTP response codes:

Health status can be queried by query parameters (OnServiceFailure, OnDispatcherFailure, OnListenerFailure, OnThrottlePercentExceeded). HTTP response code (200 – 599) can be specified for each query parameter. If the HTTP response code is omitted for a query parameter, a 503 HTTP response code is used by default.

Query parameters and examples:

  1. OnServiceFailure:
  • Example: by querying https://contoso:81/Service1?health&OnServiceFailure=450, a 450 HTTP response status code is returned when ServiceHost.State is greater than CommunicationState.Opened.
  1. OnDispatcherFailure:
  • Example: by querying https://contoso:81/Service1?health&OnDispatcherFailure=455, a 455 HTTP response status code is returned when the state of any of the channel dispatchers is greater than CommunicationState.Opened.
  1. OnListenerFailure:
  • Example: by querying https://contoso:81/Service1?health&OnListenerFailure=465, a 465 HTTP response status code is returned when the state of any of the channel listeners is greater than CommunicationState.Opened.
  1. OnThrottlePercentExceeded: Specifies the percentage that triggers the response and its HTTP response code .
  • Example: by querying https://contoso:81/Service1?health&OnThrottlePercentExceeded= 70:350,95:500, when the throttle percentage is equal or larger than 95%, 500 the HTTP response code is returned; when the percentage is equal or larger than 70% and less then 95%, 350 is returned; otherwise, 200 is returned.

Publication of service health:

After enabling the health endpoint, the service health status can be displayed either in html (by specifying the query string: https://contoso:81/Service1?health) or xml (by specifying the query string: https://contoso:81/Service1?health&Xml) formats. https://contoso:81/Service1?health&NoContent returns empty html page.

Note:

It’s best practice to always limit access to the service health endpoint. You can restrict access by using the following mechanisms:

  1. Use a different port for the health endpoint than what’s used for the other services as well as use a firewall rule to control access.
  2. Add the desirable authentication and authorization to the health endpoint binding.
WPF – Screen narrators no longer announce elements with Collapsed or Hidden visibility

Elements with Collapsed or Hidden visibility are no longer announced by the screen readers. User interfaces containing elements with a Visibility of Collapsed or Hidden can be misrepresented by screen readers if such elements are announced to the user. In .NET Framework 4.8, WPF no longer includes Collapsed or Hidden elements in the Control View of the UIAutomation tree, so that the screen readers can no longer announce these elements.

WPF – SelectionTextBrush Property for use with Non-Adorner Based Text Selection

In the .NET Framework 4.7.2 WPF added the ability to draw TextBox and PasswordBox text selection without using the adorner layer (See Here). The foreground color of the selected text in this scenario was dictated by SystemColors.HighlightTextBrush.

In the .NET Framework 4.8 we are adding a new property, SelectionTextBrush, that allows developers to select the specific brush for the selected text when using non-adorner based text selection.

This property works only on TextBoxBase derived controls and PasswordBox in WPF applications with non-adorner based text selection enabled. It does not work on RichTextBox. If non-adorner based text selection is not enabled, this property is ignored.

To use this property, simply add it to your XAML code and use the appropriate brush or binding.

The resulting text selection will look like this:

You can combine the use of SelectionBrush and SelectionTextBrush to generate any color combination of background and foreground that you deem appropriate.

WPF – High DPI Enhancements

WPF has added support for Per-Monitor V2 DPI Awareness and Mixed-Mode DPI scaling in .NET 4.8. Additional information about these Windows concepts is available here.

The latest Developer Guide for Per monitor application development in WPF states that only pure-WPF applications are expected to work seamlessly in a high-DPI WPF application and that Hosted HWND’s and Windows Forms controls are not fully supported.

.NET 4.8 improves support for hosted HWND’s and Windows Forms interoperation in High-DPI WPF applications on platforms that support Mixed-Mode DPI scaling (Windows 10 v1803). When hosted HWND’s or Windows Forms controls are created as Mixed-Mode DPI scaled windows, (as described in the “Mixed-Mode DPI Scaling and DPI-aware APIs” documentation by calling SetThreadDpiHostingBehavior and SetThreadDpiAwarenessContext API’s), it will be possible to host such content in a Per-Monitor V2 WPF application and have them be sized and scaled appropriately. Such hosted content will not be rendered at the native DPI – instead, the OS will scale the hosted content to the appropriate size.

The support for Per-Monitor v2 DPI awareness mode also allows WPF controls to be hosted (i.e., parented) under a native window in a high-DPI application. Per-Monitor V2 DPI Awareness support will be available on Windows 10 v1607 (Anniversary Update). Windows adds support for child-HWND’s to receive DPI change notifications when Per-Monitor V2 DPI Awareness mode is enabled via the application manifest.

This support is leveraged by WPF to ensure that controls that are hosted under a native window can respond to DPI changes and update themselves. For e.g.- a WPF control hosted in a Windows Forms or a Win32 application that is manifested as Per Monitor V2 – will now be able to respond correctly to DPI changes and update itself.

Note that Windows supports Mixed-Mode DPI scaling on Windows 10 v1803, whereas Per-Monitor V2 is supported on v1607 onwards.

To try out these features, the following application manifest and AppContext flags must be enabled:

  1. Enable Per-Monitor DPI in your application
      • Turn on Per-Monitor V2 in your app.manifest
    • Target .NET Framework 4.6.2 or greater

3. Set AppContext switch in your app.config

Set AppContextSwitch Switch.System.Windows.DoNotUsePresentationDpiCapabilityTier2OrGreater=false in App.Config to enable Per-Monitor V2 and Mixed-Mode DPI support introduced in .NET 4.8.

The runtime section in the final App.Config might look like this:

AppContext switches can also be set in registry. You can refer to the AppContext Class for additional documentation.

WPF – Support for UIAutomation ControllerFor property

UIAutomation’s ControllerFor property returns an array of automation elements that are manipulated by the automation element that supports this property. This property is commonly used for Auto-suggest accessibility. ControllerFor is used when an automation element affects one or more segments of the application UI or the desktop. Otherwise, it is hard to associate the impact of the control operation with UI elements. This feature adds the ability for controls to provide a value for ControllerFor property.

A new virtual method has been added to AutomationPeer:

To provide a value for the ControllerFor property, simply override this method and return a list of AutomationPeers for the controls being manipulated by this AutomationPeer:

WPF – Tooltips on keyboard access

Currently tooltips only display when a user hovers the mouse cursor over a control. In .NET Framework 4.8, WPF is adding a feature that enables tooltips to show on keyboard focus, as well as via a keyboard shortcut.

To enable this feature, an application needs to target .NET Framework 4.8 or opt-in via AppContext switch “Switch.UseLegacyAccessibilityFeatures.3” and “Switch.UseLegacyToolTipDisplay”.

Sample App.config file:

Once enabled, all controls containing a tooltip will start to display it once the control receives keyboard focus. The tooltip can be dismissed over time or when keyboard focus changes. Users can also dismiss the tooltip manually via a new keyboard shortcut Ctrl + Shift + F10. Once the tooltip has been dismissed it can be displayed again via the same keyboard shortcut.

Note: RibbonToolTips on Ribbon controls won’t show on keyboard focus – they will only show via the keyboard shortcut.

WPF – Added Support for SizeOfSet and PositionInSet UIAutomation properties

Windows 10 introduced new UIAutomation properties SizeOfSet and PositionInSet which are used by applications to describe the count of items in a set. UIAutomation client applications such as screen readers can then query an application for these properties and announce an accurate representation of the application’s UI.

This feature adds support for WPF applications to expose these two properties to UIAutomation. This can be accomplished in two ways:

New DependencyProperties SizeOfSet and PositionInSet have been added to the System.Windows.Automation.AutomationProperties namespace. A developer can set their values via XAML:

2. AutomationPeer virtual methods

Virtual methods GetSizeOfSetCore and GetPositionInSetCore have also been added to the AutomationPeer class. A developer can provide values for SizeOfSet and PositionInSet by overriding these methods:

Automatic values

Items in ItemsControls will provide a value for these properties automatically without additional action from the developer. If an ItemsControl is grouped, the collection of groups will be represented as a set and each group counted as a separate set, with each item inside that group providing its position inside that group as well as the size of the group. Automatic values are not affected by virtualization. Even if an item is not realized, it is still counted towards the total size of the set and affects the position in the set of it’s sibling items.

Automatic values are only provided if the developer is targeting .NET Framework 4.8 or has set the AppContext switch “Switch.UseLegacyAccessibilityFeatures.3” – for example via App.config file:

Closing

Please try out these improvements in the .NET Framework 4.8 and share your feedback in the comments below or via GitHub.

TypeScript 4.8

TypeScript 4.8 brings a series of correctness and consistency improvements under —strictNullChecks . These changes affect how intersection and union types work, and are leveraged in how TypeScript narrows types.

For example, unknown is close in spirit to the union type <> | null | undefined because it accepts null , undefined , and any other type. TypeScript now recognizes this, and allows assignments from unknown to <> | null | undefined .

ts
function f(x: unknown, y: <> | null | undefined)
x = y; // always worked
y = x; // used to error, now works
>

Another change is that <> intersected with any other object type simplifies right down to that object type. That meant that we were able to rewrite NonNullable to just use an intersection with <> , because <> & null and <> & undefined just get tossed away.

diff
- type NonNullable = T extends null | undefined ? never : T;
+ type NonNullable = T & <>;

This is an improvement because intersection types like this can be reduced and assigned to, while conditional types currently cannot. So NonNullable> now simplifies at least to NonNullable , whereas it didn’t before.

ts
function fooT>(x: NonNullableT>, y: NonNullableNonNullableT>>)
x = y; // always worked
y = x; // used to error, now works
>

These changes also allowed us to bring in sensible improvements in control flow analysis and type narrowing. For example, unknown is now narrowed just like <> | null | undefined in truthy branches.

ts
function narrowUnknownishUnion(x: <> | null | undefined)
if (x)
x; // <>
>
else
x; // <> | null | undefined
>
>
function narrowUnknown(x: unknown)
if (x)
x; // used to be 'unknown', now '<>'
>
else
x; // unknown
>
>

Generic values also get narrowed similarly. When checking that a value isn’t null or undefined , TypeScript now just intersects it with <> — which again, is the same as saying it’s NonNullable . Putting many of the changes here together, we can now define the following function without any type assertions.

ts
function throwIfNullableT>(value: T): NonNullableT>
if (value === undefined || value === null)
throw Error("Nullable value!");
>
// Used to fail because 'T' was not assignable to 'NonNullable'.
// Now narrows to 'T & <>' and succeeds because that's just 'NonNullable'.
return value;
>

value now gets narrowed to T & <> , and is now identical with NonNullable — so the body of the function just works with no TypeScript-specific syntax.

On their own, these changes may appear small — but they represent fixes for many many paper cuts that have been reported over several years.

For more specifics on these improvements, you can read more here.

Improved Inference for infer Types in Template String Types

TypeScript recently introduced a way to add extends constraints to infer type variables in conditional types.

ts
// Grabs the first element of a tuple if it's assignable to 'number',
// and returns 'never' if it can't find one.
type TryGetNumberIfFirstT> =
T extends [infer U extends number, . unknown[]] ? U : never;

If these infer types appear in a template string type and are constrained to a primitive type, TypeScript will now try to parse out a literal type.

ts
// SomeNum used to be 'number'; now it's '100'.
type SomeNum = "100" extends `$infer U extends number>` ? U : never;
// SomeBigInt used to be 'bigint'; now it's '100n'.
type SomeBigInt = "100" extends `$infer U extends bigint>` ? U : never;
// SomeBool used to be 'boolean'; now it's 'true'.
type SomeBool = "true" extends `$infer U extends boolean>` ? U : never;

This can now better convey what a library will do at runtime, and give more precise types.

One note on this is that when TypeScript parses these literal types out it will greedily try to parse out as much of what looks like of the appropriate primitive type; however it then checks to see if the print-back of that primitive matches up with the string contents. In other words, TypeScript checks whether the going from the string, to the primitive, and back matches. If it doesn’t see that the string can be “round-tripped”, then it will fall back to the base primitive type.

ts
// JustNumber is `number` here because TypeScript parses out `"1.0"`, but `String(Number("1.0"))` is `"1"` and doesn't match.
type JustNumber = "1.0" extends `$infer T extends number>` ? T : never;

—build , —watch , and —incremental Performance Improvements

TypeScript 4.8 introduces several optimizations that should speed up scenarios around —watch and —incremental , along with project references builds using —build . For example, TypeScript is now able to avoid spending time updating timestamps during no-op changes in —watch mode, which makes rebuilds faster and avoids messing with other build tools that might be watching for TypeScript’s output. Many other optimizations where we’re able to reuse information across —build , —watch , and —incremental have been introduced as well.

How big are these improvements? Well, on a fairly large internal codebase, we’ve seen time reductions on the order of 10%-25% on many simple common operations, with around 40% time reductions in no-change scenarios. We’ve seen similar results on the TypeScript codebase as well.

Errors When Comparing Object and Array Literals

In many languages, operators like == perform what’s called “value” equality on objects. For example, in Python it’s valid to check whether a list is empty by checking whether a value is equal to the empty list using == .

py
if people_at_home == []:
print("here's where I lie, broken inside. )
adopt_animals()

This is not the case in JavaScript, where == and === between objects (and therefore, arrays) check whether both references point to the same value. We believe that similar code in JavaScript is at best an early foot-gun for JavaScript developers, and at worst a bug in production code. That’s why TypeScript now disallows code like the following.

ts
if (peopleAtHome === [])
// ~~~~~~~~~~~~~~~~~~~
// This condition will always return 'false' since JavaScript compares objects by reference, not value.
console.log("here's where I lie, broken inside. )
adoptAnimals();
>

We’d like to extend our gratitude to Jack Works who contributed this check. You can view the changes involved here.

Improved Inference from Binding Patterns

In some cases, TypeScript will pick up a type from a binding pattern to make better inferences.

ts
declare function chooseRandomlyT>(x: T, y: T): T;
let [a, b, c] = chooseRandomly([42, true, "hi!"], [0, false, "bye!"]);
// ^ ^ ^
// | | |
// | | string
// | |
// | boolean
// |
// number

When chooseRandomly needs to figure out a type for T , it will primarily look at [42, true, «hi!»] and [0, false, «bye!»] ; but TypeScript needs to figure out whether those two types should be Array or the tuple type [number, boolean, string] . To do that, it will look for existing candidates as a hint to see whether there are any tuple types. When TypeScript sees the binding pattern [a, b, c] , it creates the type [any, any, any] , and that type gets picked up as a low-priority candidate for T which also gets used as a hint for the types of [42, true, «hi!»] and [0, false, «bye!»] .

You can see how this was good for chooseRandomly , but it fell short in other cases. For example, take the following code

ts
declare function fT>(x?: T): T;
let [x, y, z] = f();

The binding pattern [x, y, z] hinted that f should produce an [any, any, any] tuple; but f really shouldn’t change its type argument based on a binding pattern. It can’t suddenly conjure up a new array-like value based on what it’s being assigned to, so the binding pattern type has way too much influence on the produced type. On top of that, because the binding pattern type is full of any s, we’re left with x , y , and z being typed as any .

In TypeScript 4.8, these binding patterns are never used as candidates for type arguments. Instead, they’re just consulted in case a parameter needs a more specific type like in our chooseRandomly example. If you need to revert to the old behavior, you can always provide explicit type arguments.

You can look at the change on GitHub if you’re curious to learn more.

File-Watching Fixes (Especially Across git checkout s)

We’ve had a long-standing bug where TypeScript has a very hard time with certain file changes in —watch mode and editor scenarios. Sometimes the symptoms are stale or inaccurate errors that might show up that require restarting tsc or VS Code. Frequently these occur on Unix systems, and you might have seen these after saving a file with vim or swapping branches in git.

This was caused by assumptions of how Node.js handles rename events across file systems. File systems used by Linux and macOS utilize inodes, and Node.js will attach file watchers to inodes rather than file paths. So when Node.js returns a watcher object, it might be watching a path or an inode depending on the platform and file system.

To be a bit more efficient, TypeScript tries to reuse the same watcher objects if it detects a path still exists on disk. This is where things went wrong, because even if a file still exists at that path, a distinct file might have been created, and that file will have a different inode. So TypeScript would end up reusing the watcher object instead of installing a new watcher at the original location, and watch for changes at what might be a totally irrelevant file. So TypeScript 4.8 now handles these cases on inode systems and properly installs a new watcher and fixes this.

We’d like to extend our thanks to Marc Celani and his team at Airtable who invested lots of time in investigating the issues they were experiencing and pointing out the root cause. You can view the specific fixes around file-watching here.

Find-All-References Performance Improvements

When running find-all-references in your editor, TypeScript is now able to act a little smarter as it aggregates references. This reduced the amount of time TypeScript took to search a widely-used identifier in its own codebase by about 20%.

Exclude Specific Files from Auto-Imports

TypeScript 4.8 introduces an editor preference for excluding files from auto-imports. In Visual Studio Code, file names or globs can be added under “Auto Import File Exclude Patterns” in the Settings UI, or in a .vscode/settings.json file:

jsonc
// Note that `javascript.preferences.autoImportFileExcludePatterns` can be specified for JavaScript too.
"typescript.preferences.autoImportFileExcludePatterns": [
"**/node_modules/@types/node"
]
>

This can be useful in cases where you can’t avoid having certain modules or libraries in your compilation but you rarely want to import from them. These modules might have lots of exports that can pollute the auto-imports list and make it harder to navigate, and this option can help in those situations.

Correctness Fixes and Breaking Changes

Due to the nature of type system changes, there are very few changes that can be made that don’t affect some code; however, there are a few changes that are more likely to require adapting existing code.

While TypeScript strives to avoid major breaks, even small changes in the built-in libraries can cause issues. We don’t expect major breaks as a result of DOM and lib.d.ts updates, but one notable change is that the cause property on Error s now has the type unknown instead of Error .

Unconstrained Generics No Longer Assignable to <>

In TypeScript 4.8, for projects with strictNullChecks enabled, TypeScript will now correctly issue an error when an unconstrained type parameter is used in a position where null or undefined are not legal values. That will include any type that expects <> , object , or an object type with all-optional properties.

A simple example can be seen in the following.

ts
// Accepts any non-null non-undefined value
function bar(value: <>)
Object.keys(value); // This call throws on null/undefined at runtime.
>
// Unconstrained type parameter T.
function fooT>(x: T)
bar(x); // Used to be allowed, now is an error in 4.8.
// ~
// error: Argument of type 'T' is not assignable to parameter of type '<>'.
>
foo(undefined);

As demonstrated above, code like this has a potential bug — the values null and undefined can be indirectly passed through these unconstrained type parameters to code that is not supposed to observe those values.

This behavior will also be visible in type positions. One example would be:

ts
interface FooT>
x: BarT>;
>
interface BarT extends <>>

Existing code that didn’t want to handle null and undefined can be fixed by propagating the appropriate constraints through.

diff
- function foo(x: T)
+ function foo>(x: T)

Another work-around would be to check for null and undefined at runtime.

diff
function foo(x: T)
+ if (x !== null && x !== undefined)
bar(x);
+ >
>

And if you know that for some reason, your generic value can’t be null or undefined , you can just use a non-null assertion.

diff
function foo(x: T)
- bar(x);
+ bar(x!);
>

When it comes to types, you’ll often either need to propagate constraints, or intersect your types with <> .

Decorators are placed on modifiers on TypeScript’s Syntax Trees

The current direction of decorators in TC39 means that TypeScript will have to handle a break in terms of placement of decorators. Previously, TypeScript assumed decorators would always be placed prior to all keywords/modifiers. For example

ts
@decorator
export class Foo
// .
>

Decorators as currently proposed do not support this syntax. Instead, the export keyword must precede the decorator.

ts
export @decorator class Foo
// .
>

Unfortunately, TypeScript’s trees are concrete rather than abstract, and our architecture expects syntax tree node fields to be entirely ordered before or after each other. To support both legacy decorators and decorators as proposed, TypeScript will have to gracefully parse, and intersperse, modifiers and decorators.

To do this, it exposes a new type alias called ModifierLike which is a Modifier or a Decorator .

ts
export type ModifierLike = Modifier | Decorator;

Decorators are now placed in the same field as modifiers which is now a NodeArray when set, and the entire field is deprecated.

diff
- readonly modifiers?: NodeArray | undefined;
+ /**
+ * @deprecated .
+ * Use `ts.canHaveModifiers()` to test whether a `Node` can have modifiers.
+ * Use `ts.getModifiers()` to get the modifiers of a `Node`.
+ * .
+ */
+ readonly modifiers?: NodeArray | undefined;

All existing decorators properties have been marked as deprecated and will always be undefined if read. The type has also been changed to undefined so that existing tools know to handle them correctly.

diff
- readonly decorators?: NodeArray | undefined;
+ /**
+ * @deprecated .
+ * Use `ts.canHaveDecorators()` to test whether a `Node` can have decorators.
+ * Use `ts.getDecorators()` to get the decorators of a `Node`.
+ * .
+ */
+ readonly decorators?: undefined;

To avoid new deprecation warnings and other issues, TypeScript now exposes four new functions to use in place of the decorators and modifiers properties. There are individual predicates for testing whether a node has support modifiers and decorators, along with respective accessor functions for grabbing them.

ts
function canHaveModifiers(node: Node): node is HasModifiers;
function getModifiers(node: HasModifiers): readonly Modifier[] | undefined;
function canHaveDecorators(node: Node): node is HasDecorators;
function getDecorators(node: HasDecorators): readonly Decorator[] | undefined;

As an example of how to access modifiers off of a node, you can write

ts
const modifiers = canHaveModifiers(myNode) ? getModifiers(myNode) : undefined;

With the note that each call to getModifiers and getDecorators may allocate a new array.

For more information, see changes around

  • the restructuring of our tree nodes
  • the deprecations
  • exposing the predicate functions

Types Cannot Be Imported/Exported in JavaScript Files

TypeScript previously allowed JavaScript files to import and export entities declared with a type, but no value, in import and export statements. This behavior was incorrect, because named imports and exports for values that don’t exist will cause a runtime error under ECMAScript modules. When a JavaScript file is type-checked under —checkJs or through a // @ts-check comment, TypeScript will now issue an error.

ts
// @ts-check
// Will fail at runtime because 'SomeType' is not a value.
import someValue, SomeType > from "some-module";
/**
* @type
*/
export const myValue = someValue;
/**
* @typedef MyType
*/
// Will fail at runtime because 'MyType' is not a value.
export MyType as MyExportedType >;

To reference a type from another module, you can instead directly qualify the import.

diff
- import < someValue, SomeType >from "some-module";
+ import < someValue >from "some-module";
/**
- * @type
+ * @type
*/
export const myValue = someValue;

To export a type, you can just use a /** @typedef */ comment in JSDoc. @typedef comments already automatically export types from their containing modules.

diff
/**
* @typedef MyType
*/
+ /**
+ * @typedef MyExportedType
+ */
- export < MyType as MyExportedType >;

Binding Patterns Do Not Directly Contribute to Inference Candidates

As mentioned above, binding patterns no longer change the type of inference results in function calls. You can read more about the original change here.

Unused Renames in Binding Patterns are Now Errors in Type Signatures

TypeScript’s type annotation syntax often looks like it can be used when destructuring values. For example, take the following function.

ts
declare function makePerson( name: string, age: number >): Person;

You might read this signature and think that makePerson obviously takes an object with a name property with the type string and an age property with the type number ; however, JavaScript’s destructuring syntax is actually taking precedence here. makePerson does say that it’s going to take an object with a name and an age property, but instead of specifying a type for them, it’s just saying that it renames name and age to string and number respectively.

In a pure type construct, writing code like this is useless, and typically a mistake since developers usually assume they’re writing a type annotation.

TypeScript 4.8 makes these an error unless they’re referenced later in the signature. The correct way to write the above signature would be as follows:

ts
declare function makePerson(options: name: string, age: number >): Person;
// or
declare function makePerson( name, age >: name: string, age: number >): Person;

This change can catch bugs in declarations, and has been helpful for improving existing code. We’d like to extend our thanks to GitHub user uhyo for providing this check. You can read up on the change here.

Где находится датчик температуры в Audi A4 B8 — подробные сведения и местоположение

Датчик температуры — это одна из важных частей автомобиля, которая помогает контролировать и поддерживать оптимальную температуру в двигателе. Один из популярных автомобилей, в котором установлен датчик температуры, является Ауди А4 Б8. Датчик температуры в Ауди А4 Б8 находится в двигателе, и его местоположение может зависеть от конкретной модели и года выпуска автомобиля. Однако, обычно датчик температуры находится возле термостата или радиатора, так как это места, где температура двигателя и охлаждающей жидкости могут быть наиболее точно измерены и контролируемы. Зная, где находится датчик температуры Ауди А4 Б8, вы сможете легко обнаружить его при необходимости замены или обслуживания. Важно помнить, что замена датчика температуры лучше всего осуществлять после остывания двигателя, чтобы избежать опасности ожога или других повреждений.

Местонахождение датчика температуры в Ауди А4 Б8

Датчик температуры в автомобиле Audi A4 B8 расположен вблизи двигателя. Чтобы найти датчик температуры, необходимо открыть капот автомобиля. Датчик обычно размещается рядом с радиатором охлаждения двигателя. Отыскать датчик можно, обращая внимание на проводку, которая соединена с радиатором. Датчик может быть интегрирован в блок радиатора или быть установлен отдельно вблизи его верхней или боковой части. У каждой модели Audi A4 B8 местонахождение датчика может немного отличаться в зависимости от года выпуска и конкретной модификации автомобиля. Рекомендуется обратиться к руководству по эксплуатации автомобиля для получения точной информации о местонахождении датчика температуры для вашей Audi A4 B8.

Расположение датчика температуры в моторном отсеке Ауди А4 Б8

Датчик температуры в моторном отсеке Audi A4 B8 находится вблизи радиатора системы охлаждения двигателя. Конкретное расположение может немного различаться в зависимости от модификации автомобиля. Чтобы найти датчик, нужно открыть капот и обратить внимание на переднюю часть моторного отсека. Обычно датчик температуры установлен на патрубке радиатора или на блоке цилиндров рядом с ним. Он выглядит как небольшой проводок с датчиком на конце, к которому подключается разъем. Важно: перед тем, как осуществлять замену датчика температуры, рекомендуется отключить аккумулятор и остудить двигатель, чтобы предотвратить возможные травмы и повреждения. Если у вас не получается найти датчик самостоятельно или если возникли сложности при выполнении замены, рекомендуется обратиться к профессионалам или в сервисный центр Audi для получения квалифицированной помощи.

Как найти датчик температуры в салоне Ауди А4 Б8

  1. Откройте переднюю капот автомобиля.
  2. Найдите передний бампер автомобиля и обратите внимание на его нижнюю часть.
  3. Установленный на переднем бампере, в нижней части решетки радиатора, вы увидите маленькую сетчатую панель с датчиком.
  4. Осторожно нажмите на эту панель и плавно выньте датчик температуры в салоне. Обычно датчик имеет два контакта.

Теперь вы можете использовать новый датчик температуры для отображения точной температуры в салоне вашего Ауди А4 Б8.

Установка датчика температуры в системе охлаждения Ауди А4 Б8

Датчик температуры расположен в системе охлаждения двигателя ТSI на ряду с другими компонентами. Обычно он размещается на верхней части блока цилиндров или на термостате. Это позволяет ему получать точную информацию о температуре охлаждающей жидкости, передаваемой в двигатель.

Необходимые инструменты:

Для установки датчика температуры в системе охлаждения Audi A4 B8 вам понадобятся следующие инструменты:

  • гаечный ключ или насадка;
  • шестигранный ключ;
  • прокладка.

Порядок установки:

1. Отсоедините проводку от датчика температуры. Для этого необходимо слегка нажать на фиксатор и аккуратно отсоединить разъем.

2. С помощью гаечного ключа или насадки ослабьте крепление датчика. Выверните его из системы охлаждения двигателя.

3. Проверьте состояние прокладки. Если она повреждена или изношена, замените ее на новую.

4. Установите новый датчик температуры в систему охлаждения двигателя. Затяните его с помощью гаечного ключа или насадки с необходимым усилием.

5. Подсоедините проводку к датчику температуры. Убедитесь, что разъем надежно зафиксирован.

6. Проверьте плотность соединения и отсутствие утечек охлаждающей жидкости.

7. Запустите двигатель и проверьте его работу. Убедитесь, что индикатор температуры на приборной панели показывает нормальное значение.

В случае возникновения проблем или если вы не уверены в своих навыках, рекомендуется обратиться к специалистам или сертифицированному автосервису Audi для проведения установки датчика температуры в системе охлаждения.

Датчик температуры и его связь с двигателем Ауди А4 Б8

Датчик температуры обычно расположен в блоке цилиндров двигателя. Он может быть установлен в головке блока цилиндров или в блоке управления двигателем (ECU). Сигнал, полученный от датчика температуры, используется для контроля работы двигателя и эффективности его охлаждения.

Функции датчика температуры:

1. Контроль температуры двигателя: датчик температуры позволяет системе управления двигателем определить текущую температуру двигателя. Это необходимо для правильного расчета соотношения воздуха и топлива, а также для определения необходимости включения системы охлаждения.

2. Контроль работы системы охлаждения: датчик температуры помогает системе охлаждения поддерживать оптимальную температуру двигателя. Когда двигатель нагревается до определенного уровня, система охлаждения включает вентиляторы и/или насос охлаждающей жидкости для поддержания температуры в пределах допустимых значений.

Влияние датчика температуры на работу двигателя:

1. Расчет подачи топлива: информация о температуре двигателя, полученная от датчика, позволяет системе управления двигателем определить оптимальное соотношение воздуха и топлива для эффективной работы двигателя.

2. Контроль работы системы охлаждения: датчик температуры позволяет системе охлаждения эффективно управлять работой вентиляторов и насоса охлаждающей жидкости. Это позволяет поддерживать оптимальную температуру двигателя и предотвращать его перегрев.

Важно: если датчик температуры выходит из строя, это может вызвать ошибки в работе системы управления двигателем и привести к проблемам с охлаждением двигателя. Поэтому рекомендуется регулярно проверять и, при необходимости, заменять датчик температуры в соответствии с рекомендациями производителя.

Обратите внимание: данная информация относится к Ауди А4 Б8 и может отличаться для других моделей и марок автомобилей.

Как проверить работу датчика температуры Ауди А4 Б8

Датчик температуры в Ауди А4 Б8 отвечает за измерение температуры двигателя и передачу этой информации в систему управления автомобилем. Если датчик не работает должным образом, это может привести к неполадкам в системе охлаждения или привести к перегреву двигателя.

Если вам нужно проверить работоспособность датчика температуры в Ауди А4 Б8, следуйте этим шагам:

  1. Убедитесь, что автомобиль находится на плоской поверхности в выключенном состоянии и охлажденный. Это поможет избежать получение тепловых ожогов или повреждения двигателя в результате работы на горячем двигателе.
  2. Найдите датчик температуры двигателя. В случае Ауди А4 Б8 он находится на верхнем металлическом шланге радиатора или вплотную к двигателю. Обычно датчик имеет штекер и одну или две провода.
  3. Отсоедините штекер от датчика температуры и проверьте его на наличие коррозии или повреждений. При необходимости, очистите штекер или замените его.
  4. С помощью мультиметра установите его на режим измерения сопротивления и подключите один контакт мультиметра к проводу датчика и другой контакт к земле автомобиля.
  5. При проверке датчика на холодном двигателе, сопротивление должно быть около 50-100 кОм (зависит от модели). Если сопротивление отличается от заданных значений или равно бесконечности, то датчик неисправен и нуждается в замене.
  6. При проверке датчика на горячем двигателе, сопротивление должно быть около 200-400 Ом. Если сопротивление сильно отличается от заданного значения, то датчик также неисправен и требует замены.

Проверка датчика температуры Ауди А4 Б8 поможет вам определить его работоспособность и своевременно заменить его при необходимости. В случае обнаружения неисправностей, рекомендуется обратиться к специалистам для проведения ремонта или замены датчика.

Почему важно знать местоположение датчика температуры в Ауди А4 Б8

Датчик температуры является ключевым элементом системы отопления и кондиционирования автомобиля. Он отвечает за измерение температуры воздуха и передачу этой информации в электронную систему управления автомобилем. На основе показаний датчика, система регулирует работу обогревателей, кондиционеров и вентиляции, обеспечивая оптимальный комфорт для водителя и пассажиров.

Знание местоположения датчика температуры позволяет убедиться в его правильной работе и способности достоверно измерять температуру. Если датчик неисправен или находится в неправильном месте, система отопления и кондиционирования может работать неправильно или даже отказать вовсе.

Кроме того, при возникновении проблем с системой отопления или кондиционирования, знание местоположения датчика температуры позволяет произвести диагностику и определить причину неисправности. Это позволяет сэкономить время и деньги на поиск и устранение неполадок.

Информация о местоположении датчика температуры также полезна при производстве тюнинга автомобиля. Многие автолюбители предпочитают устанавливать спортивные обогреватели или изменять расположение датчика температуры для оптимизации работы системы. В этом случае, знание точного местоположения датчика позволяет более эффективно проводить модификации и добиться желаемых результатов.

Таким образом, знание местоположения датчика температуры в Ауди А4 Б8 важно для обеспечения нормальной работы системы кондиционирования и отопления автомобиля, а также для диагностики и тюнинга автомобильной электроники. Надежная и точная работа датчика температуры обеспечивает комфорт и безопасность вождения в любых погодных условиях и позволяет получить максимальное удовлетворение от эксплуатации автомобиля.

Советы по замене датчика температуры в Ауди А4 Б8

Датчик температуры в Ауди А4 Б8 отвечает за измерение температуры охлаждающей жидкости двигателя. Он отправляет сигнал на приборную панель, чтобы водитель мог отслеживать температуру двигателя.

Если вы заметили, что показания температуры на панели приборов неправильные или нестабильные, то возможно, что датчик температуры неисправен и требует замены.

Чтобы заменить датчик температуры в Ауди А4 Б8, следуйте следующим советам:

Шаг 1: Остановите двигатель и дайте ему остыть перед началом работ.
Шаг 2: Расположение датчика температуры может быть разным в зависимости от модели Ауди А4 Б8. Обычно он находится вблизи термостата или на верхней части двигателя.
Шаг 3: Отсоедините разъем датчика температуры, потянув за него осторожно.
Шаг 4: Используя подходящий инструмент, ослабьте крепежные винты, которыми фиксируется датчик температуры.
Шаг 5: Аккуратно удалите старый датчик температуры.
Шаг 6: Установите новый датчик температуры на его место и закрепите крепежными винтами.
Шаг 7: Подсоедините разъем датчика температуры.
Шаг 8: Проверьте, чтобы все было правильно установлено и закреплено.
Шаг 9: Запустите двигатель и проверьте работу нового датчика температуры. Убедитесь, что показания на приборной панели стабильные и соответствуют реальной температуре двигателя.

Замена датчика температуры в Ауди А4 Б8 не является сложной процедурой, но требует аккуратности и соблюдения указанных шагов. Если у вас возникли сомнения или затруднения, лучше обратиться к профессиональному автомеханику.

Отличия датчиков температуры в разных моделях Ауди А4 Б8

Датчик температуры в автомобиле Ауди А4 Б8 отвечает за измерение температуры охлаждающей жидкости двигателя. В разных моделях Ауди А4 Б8 встречаются различные виды датчиков температуры, которые имеют некоторые отличия.

Одним из наиболее распространенных датчиков температуры в моделях Ауди А4 Б8 является датчик температуры охлаждающей жидкости двигателя. Он обычно расположен на верхней части блока цилиндров и имеет сопряжение с системой управления двигателя. Датчик температуры в данной модели представляет собой термистор, изменение его сопротивления меняется с изменением температуры охлаждающей жидкости.

В некоторых моделях Ауди А4 Б8 также используется дополнительный датчик температуры воздуха. Этот датчик отвечает за измерение температуры воздуха, поступающего во впускную систему двигателя. Он обычно расположен во впускном коллекторе или воздушном фильтре. Датчик температуры воздуха в моделях Ауди А4 Б8 имеет тот же принцип работы, что и датчик температуры охлаждающей жидкости – изменение сопротивления термистора в соответствии с изменением температуры воздуха.

Важно помнить, что для корректной работы системы управления двигателем и обнаружения возможных неисправностей важно регулярно проверять работоспособность датчиков температуры и при необходимости производить их замену.

Подробное описание датчика температуры в Ауди А4 Б8

Датчик температуры устанавливается в зоне контакта с охлаждающей жидкостью, обычно на верхнем патрубке радиатора. Это позволяет датчику получать надежные данные о температуре охлаждающей жидкости, так как именно здесь она наиболее нагревается во время работы двигателя.

Датчик температуры в Ауди А4 Б8 имеет электрический контакт, который обеспечивает связь с электронным блоком управления двигателя. При изменении температуры охлаждающей жидкости, датчик формирует соответствующий сигнал, который передается на блок управления. Благодаря этому, электронная система может контролировать температуру двигателя и принимать решения о регулировке работы системы охлаждения.

Датчик температуры в Ауди А4 Б8 обычно представлен в виде цилиндрического корпуса с резьбовым креплением. Внутри корпуса находится термистор, который является активной частью датчика и реагирует на изменения температуры охлаждающей жидкости. Также в корпусе могут быть предусмотрены дополнительные контакты, необходимые для передачи сигнала на блок управления двигателя.

Свойство Значение
Марка автомобиля Ауди А4 Б8
Тип датчика Термистор
Место установки На верхнем патрубке радиатора
Резьбовое крепление Да
Наличие дополнительных контактов Да

В случае неисправности датчика температуры, могут возникнуть проблемы с работой системы охлаждения и контролем температуры двигателя. Поэтому рекомендуется регулярно проверять работу датчика и при необходимости заменять его.

Важно отметить, что описание датчика температуры в Ауди А4 Б8 может незначительно отличаться в зависимости от года выпуска автомобиля и его модификации. Поэтому перед покупкой нового датчика следует убедиться в его совместимости с конкретной моделью автомобиля.

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Announcing TypeScript 4.8 RC

Today we’re excited to announce our Release Candidate (RC) of TypeScript 4.8. Between now and the stable release of TypeScript 4.8, we expect no further changes apart from critical bug fixes. To get started using the RC, you can get it through NuGet, or use npm with the following command:

npm install -D typescript@rc 
  • Downloading for Visual Studio 2022/2019
  • Following directions for Visual Studio Code.

Here’s a quick list of what’s new in TypeScript 4.8!

  • Improved Intersection Reduction, Union Compatibility, and Narrowing
  • Improved Inference for infer Types in Template String Types
  • —build , —watch , and —incremental Performance Improvements
  • Errors When Comparing Object and Array Literals
  • Improved Inference from Binding Patterns
  • File-Watching Fixes (Especially Across git checkout s)
  • Find-All-References Performance Improvements
  • Exclude Specific Files from Auto-Imports
  • Correctness Fixes and Breaking Changes

What’s New Since the Beta?

Since our beta release, the RC now has support for excluding which files are considered in auto-imports. The beta release post also did not document a break around unused destructuring aliases in type signatures, which is now detailed in our Correctness Fixes and Breaking Changes section.

Improved Intersection Reduction, Union Compatibility, and Narrowing

TypeScript 4.8 brings a series of correctness and consistency improvements under —strictNullChecks . These changes affect how intersection and union types work, and are leveraged in how TypeScript narrows types.

For example, unknown is close in spirit to the union type <> | null | undefined because it accepts null , undefined , and any other type. TypeScript now recognizes this, and allows assignments from unknown to <> | null | undefined .

function f(x: unknown, y: <> | null | undefined) x = y; // always worked y = x; // used to error, now works > 

Another change is that <> intersected with any other object type simplifies right down to that object type. That meant that we were able to rewrite NonNullable to just use an intersection with <> , because <> & null and <> & undefined just get tossed away.

- type NonNullable = T extends null | undefined ? never : T; + type NonNullable = T & <>; 

This is an improvement because intersection types like this can be reduced and assigned to, while conditional types currently cannot. So NonNullable> now simplifies at least to NonNullable , whereas it didn’t before.

function fooT>(x: NonNullableT>, y: NonNullableNonNullableT>>) x = y; // always worked y = x; // used to error, now works > 

These changes also allowed us to bring in sensible improvements in control flow analysis and type narrowing. For example, unknown is now narrowed just like <> | null | undefined in truthy branches.

function narrowUnknownishUnion(x: <> | null | undefined) if (x) x; // <> > else  x; // <> | null | undefined > > function narrowUnknown(x: unknown) if (x) x; // used to be 'unknown', now '<>' > else  x; // unknown > > 

Generic values also get narrowed similarly. When checking that a value isn’t null or undefined , TypeScript now just intersects it with <> – which again, is the same as saying it’s NonNullable . Putting many of the changes here together, we can now define the following function without any type assertions.

function throwIfNullableT>(value: T): NonNullableT> if (value === undefined || value === null) throw Error("Nullable value!"); > // Used to fail because 'T' was not assignable to 'NonNullable'. // Now narrows to 'T & <>' and succeeds because that's just 'NonNullable'. return value; > 

value now gets narrowed to T & <> , and is now identical with NonNullable – so the body of the function just works with no TypeScript-specific syntax.

On their own, these changes may appear small – but they represent fixes for many many paper cuts that have been reported over several years.

For more specifics on these improvements, you can read more here.

Improved Inference for infer Types in Template String Types

TypeScript recently introduced a way to add extends constraints to infer type variables in conditional types.

// Grabs the first element of a tuple if it's assignable to 'number', // and returns 'never' if it can't find one. type TryGetNumberIfFirstT> = T extends [infer U extends number, . unknown[]] ? U : never; 

If these infer types appear in a template string type and are constrained to a primitive type, TypeScript will now try to parse out a literal type.

// SomeNum used to be 'number'; now it's '100'. type SomeNum = "100" extends `$infer U extends number>` ? U : never; // SomeBigInt used to be 'bigint'; now it's '100n'. type SomeBigInt = "100" extends `$infer U extends bigint>` ? U : never; // SomeBool used to be 'boolean'; now it's 'true'. type SomeBool = "true" extends `$infer U extends boolean>` ? U : never; 

This can now better convey what a library will do at runtime, and give more precise types.

One note on this is that when TypeScript parses these literal types out it will greedily try to parse out as much of what looks like of the appropriate primitive type; however it then checks to see if the print-back of that primitive matches up with the string contents. In other words, TypeScript checks whether the going from the string, to the primitive, and back matches. If it doesn’t see that the string can be «round-tripped», then it will fall back to the base primitive type.

// JustNumber is `number` here because TypeScript parses out `"1.0"`, but `String(Number("1.0"))` is `"1"` and doesn't match. type JustNumber = "1.0" extends `$infer T extends number>` ? T : never; 

—build , —watch , and —incremental Performance Improvements

TypeScript 4.8 introduces several optimizations that should speed up scenarios around —watch and —incremental , along with project references builds using —build . For example, TypeScript is now able to avoid spending time updating timestamps during no-op changes in —watch mode, which makes rebuilds faster and avoids messing with other build tools that might be watching for TypeScript’s output. Many other optimizations where we’re able to reuse information across —build , —watch , and —incremental have been introduced as well.

How big are these improvements? Well, on a fairly large internal codebase, we’ve seen time reductions on the order of 10%-25% on many simple common operations, with around 40% time reductions in no-change scenarios. We’ve seen similar results on the TypeScript codebase as well.

Errors When Comparing Object and Array Literals

In many languages, operators like == perform what’s called «value» equality on objects. For example, in Python it’s valid to check whether a list is empty by checking whether a value is equal to the empty list using == .

if people_at_home == []: print("that's where she lies, broken inside. ) 

This is not the case in JavaScript, where == and === between objects and arrays check whether both references point to the same instance. We believe that this is at best an early foot-gun for JavaScript developers, and at worst a bug in production code. That’s why TypeScript now disallows code like the following.

let peopleAtHome = []; if (peopleAtHome === []) // ~~~~~~~~~~~~~~~~~~~ // This condition will always return 'false' since JavaScript compares objects by reference, not value. console.log("that's where she lies, broken inside. ) > 

We’d like to extend our gratitude to Jack Works who contributed this check. You can view the changes involved here.

Improved Inference from Binding Patterns

In some cases, TypeScript will pick up a type from a binding pattern to make better inferences.

declare function chooseRandomlyT>(x: T, y: T): T; let [a, b, c] = chooseRandomly([42, true, "hi!"], [0, false, "bye!"]); // ^ ^ ^ // | | | // | | string // | | // | boolean // | // number 

When chooseRandomly needs to figure out a type for T , it will primarily look at [42, true, «hi!»] and [0, false, «bye!»] ; but TypeScript needs to figure out whether those two types should be Array or the tuple type [number, boolean, string] . To do that, it will look for existing candidates as a hint to see whether there are any tuple types. When TypeScript sees the binding pattern [a, b, c] , it creates the type [any, any, any] , and that type gets picked up as a low-priority candidate for T which also gets used as a hint for the types of [42, true, «hi!»] and [0, false, «bye!»] .

You can see how this was good for chooseRandomly , but it fell short in other cases. For example, take the following code

declare function fT>(x?: T): T; let [x, y, z] = f(); 

The binding pattern [x, y, z] hinted that f should produce an [any, any, any] tuple; but f really shouldn’t change its type argument based on a binding pattern. It can’t suddenly conjure up a new array-like value based on what it’s being assigned to, so the binding pattern type has way too much influence on the produced type. On top of that, because the binding pattern type is full of any s, we’re left with x , y , and z being typed as any .

In TypeScript 4.8, these binding patterns are never used as candidates for type arguments. Instead, they’re just consulted in case a parameter needs a more specific type like in our chooseRandomly example. If you need to revert to the old behavior, you can always provide explicit type arguments.

You can look at the change on GitHub if you’re curious to learn more.

File-Watching Fixes (Especially Across git checkout s)

We’ve had a long-standing bug where TypeScript has a very hard time with certain file changes in —watch mode and editor scenarios. Sometimes the symptoms are stale or inaccurate errors that might show up that require restarting tsc or VS Code. Frequently these occur on Unix systems, and you might have seen these after saving a file with vim or swapping branches in git.

This was caused by assumptions of how Node.js handles rename events across file systems. File systems used by Linux and macOS utilize inodes, and Node.js will attach file watchers to inodes rather than file paths. So when Node.js returns a watcher object, it might be watching a path or an inode depending on the platform and file system.

To be a bit more efficient, TypeScript tries to reuse the same watcher objects if it detects a path still exists on disk. This is where things went wrong, because even if a file still exists at that path, a distinct file might have been created, and that file will have a different inode. So TypeScript would end up reusing the watcher object instead of installing a new watcher at the original location, and watch for changes at what might be a totally irrelevant file. So TypeScript 4.8 now handles these cases on inode systems and properly installs a new watcher and fixes this.

We’d like to extend our thanks to Marc Celani and his team at Airtable who invested lots of time in investigating the issues they were experiencing and pointing out the root cause. You can view the specific fixes around file-watching here.

Find-All-References Performance Improvements

When running find-all-references in your editor, TypeScript is now able to act a little smarter as it aggregates references. This reduced the amount of time TypeScript took to search a widely-used identifier in its own codebase by about 20%.

Exclude Specific Files from Auto-Imports

TypeScript 4.8 introduces an editor preference for excluding files from auto-imports. In Visual Studio Code, file names or globs can be added under «Auto Import File Exclude Patterns» in the Settings UI, or in a .vscode/settings.json file:

 // Note that `javascript.preferences.autoImportFileExcludePatterns` can be specified for JavaScript too. "typescript.preferences.autoImportFileExcludePatterns": [ "**/node_modules/@types/node" ] > 

This can be useful in cases where you can’t avoid having certain modules or libraries in your compilation but you rarely want to import from them. These modules might have lots of exports that can pollute the auto-imports list and make it harder to navigate, and this option can help in those situations.

Correctness Fixes and Breaking Changes

Due to the nature of type system changes, there are very few changes that can be made that don’t affect some code; however, there are a few changes that are more likely to require adapting existing code.

lib.d.ts Updates

While TypeScript strives to avoid major breaks, even small changes in the built-in libraries can cause issues. We don’t expect major breaks as a result of DOM and lib.d.ts updates, but one notable change is that the cause property on Error s now has the type unknown instead of Error .

Unconstrained Generics No Longer Assignable to <>

In TypeScript 4.8, for projects with strictNullChecks enabled, TypeScript will now correctly issue an error when an unconstrained type parameter is used in a position where null or undefined are not legal values. That will include any type that expects <> , object , or an object type with all-optional properties.

A simple example can be seen in the following.

// Accepts any non-null non-undefined value function bar(value: <>) Object.keys(value); // This call throws on null/undefined at runtime. > // Unconstrained type parameter T. function fooT>(x: T) bar(x); // Used to be allowed, now is an error in 4.8. // ~ // error: Argument of type 'T' is not assignable to parameter of type '<>'. > foo(undefined); 

As demonstrated above, code like this has a potential bug – the values null and undefined can be indirectly passed through these unconstrained type parameters to code that is not supposed to observe those values.

This behavior will also be visible in type positions. One example would be:

interface FooT> x: BarT>; > interface BarT extends <>> 

Existing code that didn’t want to handle null and undefined can be fixed by propagating the appropriate constraints through.

- function foo(x: T) + function foo>(x: T) 

Another work-around would be to check for null and undefined at runtime.

 function foo(x: T) + if (x !== null && x !== undefined)  bar(x); + > > 

And if you know that for some reason, your generic value can’t be null or undefined , you can just use a non-null assertion.

 function foo(x: T) - bar(x); + bar(x!); > 

When it comes to types, you’ll often either need to propagate constraints, or intersect your types with <> .

Types Cannot Be Imported/Exported in JavaScript Files

TypeScript previously allowed JavaScript files to import and export entities declared with a type, but no value, in import and export statements. This behavior was incorrect, because named imports and exports for values that don’t exist will cause a runtime error under ECMAScript modules. When a JavaScript file is type-checked under —checkJs or through a // @ts-check comment, TypeScript will now issue an error.

// @ts-check // Will fail at runtime because 'SomeType' is not a value. import  someValue, SomeType > from "some-module"; /** * @type  */ export const myValue = someValue; /** * @typedef MyType */ // Will fail at runtime because 'MyType' is not a value. export  MyType as MyExportedType >; 

To reference a type from another module, you can instead directly qualify the import.

- import < someValue, SomeType >from "some-module"; + import < someValue >from "some-module"; /** - * @type + * @type */ export const myValue = someValue; 

To export a type, you can just use a /** @typedef */ comment in JSDoc. @typedef comments already automatically export types from their containing modules.

 /** * @typedef MyType */ + /** + * @typedef MyExportedType + */ - export < MyType as MyExportedType >; 

Binding Patterns Do Not Directly Contribute to Inference Candidates

As mentioned above, binding patterns no longer change the type of inference results in function calls. You can read more about the original change here.

Unused Renames in Binding Patterns are Now Errors in Type Signatures

TypeScript’s type annotation syntax often looks like it can be used when destructuring values. For example, take the following function.

declare function makeCar( name: string, age: number >): Person; 

You might read this signature and thing that makePerson obviously takes an object with a name property with the type string and an age property with the type number ; however, JavaScript’s destructuring syntax is actually taking precedence here. makePerson does say that it’s going to take an object with a name and an age property, but instead of specifying a type for them, it’s just saying that it renames name and age to string and number respectively.

In a pure type construct, writing code like this is useless, and typically a mistake since developers usually assume they’re writing a type annotation.

TypeScript 4.8 makes these an error unless they’re referenced later in the signature. The correct way to write the above signature would be as follows:

declare function makePerson(options:  name: string, age: number >): Person; // or declare function makePerson( name, age >:  name: string, age: number >): Person; 

This change can catch bugs in declarations, and has been helpful for improving existing code. We’d like to extend our thanks to GitHub user uhyo for providing this check. You can read up on the change here.

What’s Next?

In the coming weeks, our team will be focusing on work for TypeScript 4.9, while only making critical fixes for TypeScript 4.8’s stable release. You can keep track of our target dates on the TypeScript 4.8 Iteration Plan.

In the meantime, download the Release Candidate today, and if you run into anything, give us your feedback so we can make TypeScript 4.8 even better!

– Daniel Rosenwasser and the TypeScript Team

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