A Proposal for Type Syntax in JavaScript(devblogs.microsoft.com)

    const options = Object.entries(MyEnum).map(([key, label]) => { key, label })

    // TypeScript
    function add(a: number, b: number): number { return a + b; }

    // tsc output (JavaScript)
    function add(a, b) { return a + b; }

    // (ab)use by someone using my lib without TypeScript
    add("abc", "def");

    function add(a: number, b: number): number {
        if (typeof a !== 'number' || typeof b !== 'number')
            throw 'no';
        return a + b;
    }