TypeScript - Arrow Functions

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In this tutorial we will learn about arrow functions in TypeScript.

What is an Arrow Function?

An Arrow Function a.k.a. Fat Arrow Function, are concise way of writing a function expression.

In the following example we have two ways of writing a function in ES5 and ES6 style of coding.

// ES5
var getResult = function(username, points) {
  return username + ' scored ' + points + ' points!';
};

// ES6
var getResult = (username: string, points: number): string => {
  return `${ username } scored ${ points } points!`;
}

Arrow function with no parameters and no return value

To create an arrow function with no parameters and no return value we use the given syntax.

() => {}

In the following example we have a function fun1 that takes no arguments and returns no value.

var fun1 = () => {
  console.log("Hello World");
};

// calling
fun1();

The above code is similar to the following.

var fun1 = function() {
  console.log("Hello World");
};

Arrow function with no parameters but returns some value

Following is the syntax of an arrow function having no parameters but returns some value.

(): type => {
  return expr;
}

In the following example we have a function fun2 that takes no argument but returns a value of type number.

var fun2 = (): number => {
  return Math.random();
};

// calling
console.log("Random number: " + fun2());

The above code is similar to the following code.

var fun2 = function() {
  return Math.random();
};

Arrow function with parameters but no return value

Following is the syntax of an arrow function having some parameters but returns no value.

(v1: type, v2: type, ...): void => {}

In the following example we have a function fun3 that takes two arguments (string and number) and returns no value.

var fun3 = (username: string, points: number): void => {
  let str = `${ username } scored ${ points } points!`;
  console.log(str);
};

// calling
fun3('Yusuf Shakeel', 10);

The above code is similar to the following.

var fun3 = function(username, points) {
  var str = username + ' scored ' + points + ' points!';
  console.log(str);
};

Arrow function with parameters and returns some value

Following is the syntax of an arrow function having some parameters and returning some value.

(v1: type, v2: type, ...): type => {
  return expr;
}

In the following example we have a function fun4 that takes two arguments (string and number) and returns a string value.

var fun4 = (username: string, points: number): string => {
  return `${ username } scored ${ points } points!`;
};

// calling
console.log(fun4('Yusuf Shakeel', 10));

The above code is similar to the following.

var fun4 = function(username, points) {
  return username + ' scored ' + points + ' points!';
};
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