How to write a TypeScript overload that specifies either no argument or all?

Question

I want type a function in such a way that it can be called with either no arguments or all arguments (two in my case).

I tried this:

function foo(a: string, b: number): string
function foo(a?: string, b?: number): string
function foo(a?: string, b?: number): string {
    return a + (b ? b.toString() : '')
}

…but I can still provide just one argument and receive no errors:

foo();            // ok
foo('hello', 42); // ok
foo('oops');      // should error, but doesn't

Here's a TypeScript playground link.

Is this possible?

Answer 1

function foo(a: string, b: number): string
function foo(): string // YOU NEED NO ARGS YOU WRITE NO ARGS
function foo(a?: string, b?: number): string {
    return a + (b ? b.toString() : '')
}