TypeScript function overloads

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Hello 👋🏻 Today I want to write a little bit about functions overloads in TypeScript. I will explain what is function overload and how to do it. In the next section, I will cover why you may want to overload your functions and what is a proper way of doing it.

During this article, I assume that you have at least basic knowledge of TypeScript.


Function overloads is a way of telling TypeScript that this function may take different arguments. Let’s jump into an example:

interface Data {
  postalCodes: string[];
  country: string;

const data: Data = {
  postalCodes: ["123", "422"],
  country: "PL",

This is a bit contrived example but it illustrates the point. I have an object data which fulfills interface Data. This object has two keys postalCodes & country which are string[] and string type respectively.

Below I have getDataByKey which is a helper to get either postalCodes or country.

function getDataByKey(data: Data, key: "postalCodes" | "country") {
  return data[key];

const postalCodesRetrieved: string[] = getDataByKey(data, "postalCodes");

Everything looks nice so far but at the last line I want my postalCodesRetrieved to be array of string. Yet compiler will return and error:

Type 'string | string[]' is not assignable to type 'string[]'.
  Type 'string' is not assignable to type 'string[]'.

You can find a playground with this problem under this link.

Why & How

How to fix it? You can use function overloading:

function getDataByKey(data: Data, key: "postalCodes"): string[];
function getDataByKey(data: Data, key: "country"): string;
function getDataByKey(data: Data, key: "postalCodes" | "country") {
  return data[key];

I write two overloads for getDataByKey: one is taking country as a key and returns string. Another one takes postalCodes and returns string[]. Thanks to that I can use getDataByKey with both keys:

const postalCodesRetrieved: string[] = getDataByKey(data, "postalCodes");
const countryCodesRetrieved: string = getDataByKey(data, "country");

You can even see that this function is overloaded by hovering:

Overloaded function on hover

Link to TypeScript playground with code from above.

Function overloads in an arrow function

interface GetData {
  (data: Data, key: "postalCodes"): string[];
  (data: Data, key: "country"): string;

const getData: GetData = (data, key) => {
  return data[key];

const postalCodesRetrieved: string[] = getData(data, "postalCodes");
const counryRetrieved: string = getData(data, "country");

This is working by GetData interface where overload is happening. Thanks to that I don’t need to type data & key arguments anymore. Playground link

Function overloads in class methods

class DataGetter {
  getData(data: Data, key: "country"): string;
  getData(data: Data, key: "postalCodes"): string[];
  getData(data: Data, key: "postalCodes" | "country") {
    return data[key];

const dataGetter = new DataGetter();

const postalCodesRetrieved: string[] = dataGetter.getData(data, "postalCodes");
const counryRetrieved: string = dataGetter.getData(data, "country");

It looks almost the same as for normal function but here getData method is overloaded. Playground link

Summary & TL;DR

In this post, I presented a way of telling TypeScript that the same function can be called with different parameters or returns different results. This is a function overloading. You can overload class methods too. In the simplest example, you need to write a declaration of override function at the top of a function definition.

Do you use it? If so where? Write to me at krzysztof.zuraw(at)fastmail.com.

You can find all playgrounds links here: