Type Predicates

Type Predicates in TypeScript are functions that return a boolean value and are used to narrow the type of a variable to a more specific type.

const isString = (value: unknown): value is string => typeof value === 'string';

const foo = (bar: unknown) => {
    if (isString(bar)) {
        console.log(bar.toUpperCase());
    } else {
        console.log('not a string');
    }
};

TypeScript 5.5 automatically infers type predicates (like x is T) in functions such as .filter, so it knows when values like undefined are removed—giving more precise types and fewer errors; this works for clear checks (e.g., x !== undefined) but not ambiguous ones like !!x.

const nums = [1, null, 2].filter(x => x !== null);