TypeScript - Functions

In this tutorial we will learn to work with functions in TypeScript.

In JavaScript if we want to create a function that will print "Hello World" we write the following code.

function greetings() {
  console.log("Hello World");
}

// calling
greetings();

The above code is also valid in TypeScript and it will also give us the same result.

Now we will add types and enhance the functions.

Function with no return type

If a function is not going to return any value then we can set the return type to void.

function greetings(): void {
  console.log("Hello World");
}

// calling
greetings();

Function returning some value

Following is the syntax of a function returning value.

function funName(): type {
  // some code...
}

Where, funName is the name of the function and it returns a value of given type.

In the following example the function getNumber returns value of type number.

function getNumber(): number {
  return Math.random();
}

// calling
console.log("Random number: " + getNumber());

Function with parameters

Following is the syntax of a function with parameters.

function funName(val1: type): retType {
  // some code...
}

Where, funName is the name of the function. val1 is a parameter of given type. And the return type of the function is retType.

In the following example we have an addition function which takes two numbers as argument and returns a number.

function addition(x: number, y: number): number {
  return x + y;
}

// calling
console.log("Sum: 1 + 2 = " + addition(1, 2));

Note! If we pass value that is not of number type to the above addition() function then we will get an error.

Function type

We can use function signature (parameter list and return type) to create a new type.

In the following example we are creating a variable myFun of function type using Arrow Function.

let myFun: (v1: number, v2: number) => number;

So, the above line tells us that myFun can only be assigned a function that has two parameters of type number and the return type of the function is number.

So, lets create a function and then assign it to the myFun variable.

// function
function multiply(x: number, y: number): number {
  return x * y;
}

// variable of function type
let myFun: (v1: number, v2: number) => number;

// assign function
myFun = multiply;

// now call the function
console.log("Multiply 1 x 2 = " + myFun(1, 2));