♻️ TypeScript function overloading

In this example I used ow for runtime arguments type checking, but you can check yourself typeof a === 'string'

import ow from "ow";

function myMethod(a: string): string;
function myMethod(a: number): number;
function myMethod(a: number, b: string): [number, string];
function myMethod(
	a: string | number,
	b?: string,
): string | number | [number, string] {
	if (ow.isValid(a, ow.string) && ow.isValid(b, ow.undefined)) {
		console.log("function myMethod(a: string): string");
		return a;
	}

	if (ow.isValid(a, ow.number) && ow.isValid(b, ow.undefined)) {
		console.log("function myMethod(a: number): number");
		return a;
	}

	if (ow.isValid(a, ow.number) && ow.isValid(b, ow.string)) {
		console.log("function myMethod(a: number, b: string): [number, string]");
		return [a as number, b];
	}
}

console.log(myMethod(123));
// function myMethod(a: number): number
// 123

console.log(myMethod("Andrew"));
// function myMethod(a: string): string
// Andrew

console.log(myMethod(123, "Andrew"));
// function myMethod(a: number, b: string): [number, string]
// [123, "Andrew"]

Same is valid for classes

import ow from "ow";

class MyClass {
	myMethod(a: string): string;
	myMethod(a: number): number;
	myMethod(a: number, b: string): [number, string];
	myMethod(a: string | number, b?: string): string | number | [number, string] {
		if (ow.isValid(a, ow.string) && ow.isValid(b, ow.undefined)) {
			console.log("function myMethod(a: string): string");
			return a;
		}

		if (ow.isValid(a, ow.number) && ow.isValid(b, ow.undefined)) {
			console.log("function myMethod(a: number): number");
			return a;
		}

		if (ow.isValid(a, ow.number) && ow.isValid(b, ow.string)) {
			console.log("function myMethod(a: number, b: string): [number, string]");
			return [a as number, b];
		}
	}
}

const myClass = new MyClass();

console.log(myClass.myMethod(123));
// function myMethod(a: number): number
// 123

console.log(myClass.myMethod("Andrew"));
// function myMethod(a: string): string
// Andrew

console.log(myClass.myMethod(123, "Andrew"));
// function myMethod(a: number, b: string): [number, string]
// [123, "Andrew"]

https://codesandbox.io/embed/l7k6p45589