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TypeScript 3.1

Mapped types on tuples and arrays

In TypeScript 3.1, mapped object types[1] over tuples and arrays now produce new tuples/arrays, rather than creating a new type where members like push(), pop(), and length are converted. For example:

type MapToPromise<T> = { [K in keyof T]: Promise<T[K]> };
type Coordinate = [number, number];
type PromiseCoordinate = MapToPromise<Coordinate>; // [Promise<number>, Promise<number>]

MapToPromise takes a type T, and when that type is a tuple like Coordinate, only the numeric properties are converted. In [number, number], there are two numerically named properties: 0 and 1. When given a tuple like that, MapToPromise will create a new tuple where the 0 and 1 properties are Promises of the original type. So the resulting type PromiseCoordinate ends up with the type [Promise<number>, Promise<number>].

Properties declarations on functions

TypeScript 3.1 brings the ability to define properties on function declarations and const-declared functions, simply by assigning to properties on these functions in the same scope. This allows us to write canonical JavaScript code without resorting to namespace hacks. For example:

function readImage(path: string, callback: (err: any, image: Image) => void) {
// ...
readImage.sync = (path: string) => {
const contents = fs.readFileSync(path);
return decodeImageSync(contents);

Here, we have a function readImage which reads an image in a non-blocking asynchronous way. In addition to readImage, we’ve provided a convenience function on readImage itself called readImage.sync.

While ECMAScript exports are often a better way of providing this functionality, this new support allows code written in this style to “just work” TypeScript. Additionally, this approach for property declarations allows us to express common patterns like defaultProps and propTypes on React function components (formerly known as SFCs).

export const FooComponent = ({ name }) => <div>Hello! I am {name}</div>;
FooComponent.defaultProps = {
name: "(anonymous)",

[1] More specifically, homomorphic mapped types like in the above form.

Version selection with typesVersions

Feedback from our community, as well as our own experience, has shown us that leveraging the newest TypeScript features while also accommodating users on the older versions are difficult. TypeScript introduces a new feature called typesVersions to help accommodate these scenarios.

You can read about it in the Publishing section of the declaration files section

The TypeScript docs are an open source project. Help us improve these pages by sending a Pull Request

Contributors to this page:
DRDaniel Rosenwasser  (51)
OTOrta Therox  (18)
MUMasato Urai  (1)
SSSebastian Silbermann  (1)
VKVeniamin Krol  (1)

Last updated: May 10, 2021