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typeGuard

Aims at making the most of the value is T statement.
Implementation:
export function typeGuard<T>(value: any, isMatched: boolean): value is T {
return isMatched;
}

Use case 1: Tell the compiler what assertion can be made on a given variable if a given test returns true.

import { typeGuard } from "tsafe/typeGuard";
​
type Circle = { type: "CIRCLE"; radius: number };
type Square = { type: "SQUARE"; sideLength: number };
type Shape = Circle | Square;
​
declare const shape: Shape;
​
if (typeGuard<Circle>(shape, shape.type.startsWith("C"))) {
//The developer knows the shape is is a CIRCLE,
//TypeScript can't tell but trusts the developer.
shape.radius;
} else {
shape.sideLength;
}

Use case 2: Helper for safely build other type guards

import { typeGuard } from "tsafe/typeGuard";
type SetLike<T> = { values: () => {} };
​
export function matchSetLike<T>(set: any): set is SetLike<T> {
return (
set instanceof Object &&
typeGuard<SetLike<T>>(set, true) &&
typeof set.values === "function" &&
/Set/.test(Object.getPrototypeOf(set).constructor.name)
);
}