TypeScript 4.0 Adds Long-Awaited Variadic Tuple Types

TypeScript 4.0 Adds Long-Awaited Variadic Tuple Types

The TypeScript team announced the release of TypeScript 4.0, which includes long-awaited variadic tuple type support and other improvements without introducing any major breaking changes.

The TypeScript team announced the release of TypeScript 4.0, which includes long-awaited variadic tuple type support and other improvements without introducing any major breaking changes.

For years, TypeScript lead architect Anders Hjelsberg has talked about the challenges of adding support for variadic kinds. For tuple types, it's easy to quickly run into challenges where typing something as simple as the concatenation of two array or tuple types to make a new array quickly leads to highly verbose type overloads.

TypeScript program manager Daniel Rosenwasser explains:

TypeScript 4.0 brings two fundamental changes, along with inference improvements, to make typing these possible.

The first change is that spreads in tuple type syntax can now be generic. This means that we can represent higher-order operations on tuples and arrays even when we don’t know the actual types we’re operating over. When generic spreads are instantiated (or, replaced with a real type) in these tuple types, they can produce other sets of array and tuple types. The second change is that rest elements can occur anywhere in a tuple – not just at the end!

Together these changes take a case of "death by a thousand overloads" into a relatively simple function signature:

type Arr = readonly any[];

function concat<T extends Arr, U extends Arr>(arr1: T, arr2: U): [...T, ...U] {
    return [...arr1, ...arr2];
}

Beyond this example, the type inference process for rest parameters and rest type elements eases support for partially applying arguments. Variadic tuple types generally provide significant improvements for function composition.

Improvements to tuple types and parameter lists also allow strong type validation for other common JavaScript practices that were previously challenging to type. These include manipulating argument lists and passing them to other functions.

Another significant addition in TypeScript 4.0 is class property inference from constructors. TypeScript's control flow analysis determines the types of properties in classes where the noImplicitAny flag is enabled.

TypeScript 4.0 supports a new ECMAScript feature with three new assignment operators: &&=||=, and ??=. These operators support compound assignment (e.g. the more familiar +=) for logical and nullish coalescing operators, filling in a missing gap in JavaScript and TypeScript.

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