How do function overloads work?

An overloaded function has one or more signature declarations followed by a single implementation. Callers only see the signatures, not the implementation signature. That lets you describe precise input and output relationships while keeping one runtime function.

function format(value: string): string;
function format(value: number): string;
function format(value: string | number): string {
  return typeof value === "number"
    ? value.toFixed(2)
    : value.trim();
}

format("  hello  ");
format(3.14159);

Rules to remember

• The implementation signature must be compatible with every overload
• The implementation signature is not visible to callers
• Order matters: more specific overloads should come first
• The implementation can be broader than any single overload
• Overloads cannot be expressed with a single arrow function declaration

Overloads are useful when the return type depends on the argument type or when a function accepts a few distinct call shapes. For a simple union that always returns the same type, a single signature is easier to read and maintain.