Typescript 5.4 Beta Released with Noinfer Utility Type and Improved Type Inference

Daniel Rosenwasser, senior program manager for TypeScript, recently announced the beta release of TypeScript 5.4. TypeScript 5.4 features improved type inference, a new NoInfer utility type, and breaking changes.

TypeScript 5.4 beta improved type inference for some types that are narrowed in closures.

function getUrls(url: string | URL, names: string[]) {
  if (typeof url === "string") {
    url = new URL(url);
  }

  return names.map(name => {
    url.searchParams.set("name", name)
    // ~~~~~~~~~~~~
    // error!
    // Property 'searchParams' does not    exist on type 'string | URL'.  

    return url.toString();
  });
}

In the code above, the url variable inside the map's arrow function’s closure is necessarily of type URL. Previous versions of TypeScript would infer url's type as string | URL as in the getUrls function’s signature. The released beta correctly understands that the arrow function is always created after the last assignment to url of an URL object and thus infers url as having type URL.

Some developers welcomed the improved inference on Twitter.

If those type narrowing changes are as good as they seem, that’s a huge improvement and will save a lot of unneeded if statements, ! operators, and more. (Source)

Preserved Narrowing in Closures is going to be very helpful, thank you devs. (Source)

For more detail, developers can refer to the release note (Preserved Narrowing in Closures Following Last Assignments).

TypeScript 5.4 also adds declarations for JavaScript’s new Object.groupBy and Map.groupBy static methods. The release note warns:

Note also that these methods are only accessible by configuring your target to esnext or adjusting your lib settings. We expect they will eventually be available under a stable es2024 target.

TypeScript additionally provide the new NoInfer utility type for developers to opt out TypeScript’s type inference. NoInfer allows developers to block inferences to valid but unwanted types. It is particularly useful when working with generic type parameters.

function createStreetLight<C extends string>(colors: C[], defaultColor?: NoInfer<C>) {
  // ...
}

createStreetLight(["red", "yellow", "green"], "blue");
// ~~~~~~
// error!
// Argument of type '"blue"' is not assignable to parameter of type 
// '"red" | "yellow" | "green" | undefined'.

In the previous code sample, TypeScript 5.4 assigns C as type of optional parameter defaultColor but does not pursue its inference further, ultimately resolving C type as "red" | "yellow" | "green". Previous versions of TypeScript (which do not have the NoInfer utility type) infer the type of C as "red" | "yellow" | "green" | "blue".

Developers can review the full release note on Microsoft’s developer blog, including the breaking changes resulting from the newly improved accuracy. The release note includes additional technical details, examples, and the full list of features part of the release.

Developers can install the released beta as follows:

npm install -D typescript@beta