Tips for restricting custom number types based on if conditions in TypeScript

Programming Digits

type NumType = 1 | 2 | 3 | 4 | 5;

function numFunc(number: NumType) {
    console.log("Number selected: ", number);
}

const randomNumber = Math.floor(Math.random() * 10);
if (0 < randomNumber && randomNumber < 6) {
    numFunc(randomNumber)
}

When dealing with this code, TypeScript throws an error

Argument of type 'number' is not assignable to parameter of type 'NumType'

Is there a way to enforce that TypeScript compiler confirms that randomNumber is of type NumType ?

Answer №1

One effective solution to tackle this problem is by utilizing a type guard.

const isFingerValid = (n: number): n is FingerType => 0 < n && n < 6

const randomNumber = Math.floor(Math.random() * 10);
if (isFingerValid(randomNumber)) {
    fingerFunction(randomNumber)
}

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