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TypeScript function overloading

iamandrewluca profile image Andrew Luca ・2 min read

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

Type completion

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"]


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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"]

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https://codesandbox.io/embed/l7k6p45589

Discussion (1)

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colinbradley profile image
Colin Bradley

Nice - I didn't know about that!

I neatened up your example a little: diffchecker.com/FuL9OkcM