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is is meant to be used in conjunction with assert and enable you to tell the compiler:
"Trust me this value is of type T" or "Trust me this value is not of type T"
import { assert } from "tsafe/assert";
import { is } from "tsafe/is";
type Circle = { radius: number };
type Square = { sideLength: number };
type Shape = Circle | Square;
declare const shape: Shape;
//You: Trust me TypeScript, I know that shape is a Circle.
//TypeScript: Ok if you say so...it has a radius then.
Equally useful you can tell TypeScript that your shape is not a Square, it will infer that, it is then a Circle.
//You: Trust me TypeScript, I know that shape is not a Square.
//TypeScript: Ok so by elimination it should be a Circle!
is must always be used in conjunction with assert as described in the example above.
You aren't even allowed to do something like assert(is<Circle>(shape) && shape.radius > 100 )
For any other use case considertypeGuard instead.
It is important to understand that here that when you run the instruction assert(typeGuard<Circle>(shape)) if the shape happens not to be a Circle you won't get an error at runtime.
Last modified 10mo ago