The identity function
Literally just:
export const id = <T>(x: T) => x;
It directly returns the parameter it was given as input.

Example 1: Simultaneously declaring a type and instantiating a default value for this type

import { id } from "tsafe/id";
const defaultCat = {
name: "Felix",
gender: id<"male" | "female">("male"),
type Cat = typeof defaultCat;
Cat["gender"] is "male" | "female"
If we don't use id, Cat["gender"] is of type string
Cat["gender"] is string
We could have used "male" as "male" | "female"
const defaultCat = {
name: "Felix",
gender: "male" as "male" | "female",
type Cat = typeof defaultCat;
But this is less type safe because we do not validate that the value that we gives to gender is actually assignable to "male" | "female".
This error for example slips through:
"MALE" is all caps, which should be a typing error

Example 2: Instantiating an object of type T

Let's say you have this function:
declare function getArea(shape: Shape): number;
And let's say a shape object is defined as follows:
type Circle = { type: "circle"; radius: number };
type Square = { type: "square"; sideLength: number };
type Shape = Circle | Square;
We want to instantiate a Circle and pass it to getArea we can do:
const circle: Circle = { type: "circle", radius: 33 };
If we want to avoid declaring a variable, we can do
getArea({ type: "circle", radius: 33 });
The problem, however, is that this Circle was not as easy to instantiate because TypeScript doesn’t know what kind of shape we are trying to instantiate:
Every possible properties are listed
id lets you declare that the shape you are instantiating is a Circle
import { id } from "tsafe/id";
getArea(id<Circle>({ type: "circle", radius: 33 }));
TypeScript knows we are instantiating a Circle
Last modified 2mo ago