Type Guards and Narrowing

Narrowing is TypeScript refining a union type to one specific possibility, based on a runtime check you've already written.

function printLength(value: string | string[]) {
    if (typeof value === "string") {
        console.log(value.length);       // narrowed to string
    } else {
        console.log(value.length);       // narrowed to string[]
    }
}

typeof, instanceof, and in are the most common type guards. Once you've checked a condition, TypeScript remembers it for the rest of that code branch — no extra syntax needed beyond the check itself.