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We’ve had a long-standing bug where TypeScript has a very hard time with certain file changes in -watch mode and editor scenarios. File-Watching Fixes (Especially Across git checkouts) You can look at the change on GitHub if you’re curious to learn more. If you need to revert to the old behavior, you can always provide explicit type arguments. Instead, they’re just consulted in case a parameter needs a more specific type like in our chooseRandomly example. In TypeScript 4.8, these binding patterns are never used as candidates for type arguments.

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On top of that, because the binding pattern type is full of anys, we’re left with x, y, and z being typed as any. 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. The binding pattern hinted that f should produce an tuple īut f really shouldn’t change its type argument based on a binding pattern.

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You can see how this was good for chooseRandomly, but it fell short in other cases.įor example, take the following code declare function f ( x ?: T ): T let = f () When TypeScript sees the binding pattern, it creates the type, and that type gets picked up as a low-priority candidate for T which also gets used as a hint for the types of and. To do that, it will look for existing candidates as a hint to see whether there are any tuple types. When chooseRandomly needs to figure out a type for T, it will primarily look at and īut TypeScript needs to figure out whether those two types should be Array or the tuple type. if people_at_home = : print ( "that's where she lies, broken inside. In many languages, operators like = perform what’s called "value" equality on objects.įor 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 =. Errors When Comparing Object and Array Literals You can see the changes, along with the performance results on GitHub. We’ve seen similar results on the TypeScript codebase as well. 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. Many other optimizations where we’re able to reuse information across -build, -watch, and -incremental have been introduced as well. TypeScript 4.8 introduces several optimizations that should speed up scenarios around -watch and -incremental, along with project references builds using -build.įor 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. build, -watch, and -incremental Performance Improvements You can see more about this feature here. These changes affect how intersection and union types work, and are leveraged in how TypeScript narrows types.įor example, unknown is close in spirit to the union type ` ? T : never

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TypeScript 4.8 brings a series of correctness and consistency improvements under -strictNullChecks. Improved Intersection Reduction, Union Compatibility, and Narrowing

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To get started using the beta, you can get it through NuGet, or- use npm with the following command: npm install -D can also get editor support by Today we’re announcing our beta release of TypeScript 4.8!