Functions in typescript


TypeScript lets you write type-safe functions.

This post will show how to write functions, specify

parameters and return types, and define function  Signatures.


  • In TypeScript, we can specify the type for each parameter
  • The TypeScript compiler will error if we attempt to pass an incorrect type to a function
  • The syntax for defining types for function parameters is simple, just a colon  colon “:” and name of the type.

Like JavaScript, you can also use the arrow syntax to define functions.

const joinStrings =

(str1: string, str2: string) => {

return str1 + str2;



  • Typescript can often infer a function’s return type based on the code inside the function, so it’s common not to specify them explicitly.
  • To specify the function return type, we add a colon and the type right after  the parameter list, before the function body.

If there is no return type, your function can specify a ‘void” return type

function add

(x: number, y: number): number {

return x + y;


const logAdd =

(x: number, y: number) : void => {

console. log(x + y);



We can indicate that parameters are optional by adding a question mark.

Optional parameters will appear as an ‘undefined’ type if not specified.

function fullName

(first:string, last : string, middle? : string) {

if (typeof middle == ‘undefined’) {

return ‘${first} ${last}’;


else {

return ‘${first} ${middle} ${last}’ ;




Functions are a first-class property in TypeScript and can also be defined as types. Imagine a function addNumber that looks like this.

const add =

(a: number, b: number): number => {

return a + b;


This function shape or ‘signature’ can be defined as a type‚Ķ

which allows functions to be passed as a parameter and type- checked

type Add FunctionType =

(a: number, b: number) => number;

const logAdd =

(add Function : AddFunctionType): void => {

console. log( add Function ( 1, 2));

logAdd( (a: number, b: number) => {

return a+ b;


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