Typeof Guard

typeof checks narrow primitive unions before code uses type-specific operations.

typeof.ts

function describeValue(value: string | number): string {
    if (typeof value === "number") {
        return `number=${value * 2}`;
    }
    return `text=${value.toUpperCase()}`;
}

const useNumber: boolean = ;
const input: string | number = useNumber ? 7 : "ts";
const result: string = describeValue(input);

console.log(result);

function describeValue(value: string | number): string {
    if (typeof value === "number") {
        return `number=${value * 2}`;
    }
    return `text=${value.toUpperCase()}`;
}

const useNumber: boolean = ;
const input: string | number = useNumber ? 7 : "ts";
const result: string = describeValue(input);

console.log(result);

typeof guard `typeof value === "number"` tells TypeScript that the guarded branch has a number.