Specialized overloading signature

We can use a specialized signature to create multiple methods with the same name and number of parameters, but a different return type. To create a specialized signature, we must indicate the type of function parameter using a string. The string literal is used to identify which of the function overloads is invoked:

interface Document {
    createElement(tagName: "div"): HTMLDivElement; // specialized
    createElement(tagName: "span"): HTMLSpanElement; // specialized
    createElement(tagName: "canvas"): HTMLCanvasElement; // specialized
    createElement(tagName: string): HTMLElement; // non-specialized
}

In the preceding example, we have declared three specialized overloaded signatures and one non-specialized signature for the function named createElement.

When we declare a specialized signature in an object, it must be assignable to at least one non-specialized signature in the same object. This can be observed in the preceding example, as the createElement property belongs to a type that contains three specialized signatures, all of which are assignable to the non-specialized signature in the type.

When writing overloaded declarations, we must list the non-specialized signature last.