freeCodeCamp/guide/english/javascript/arrow-functions/index.md

title
Arrow Functions

Arrow functions are a new ES6 syntax for writing JavaScript function expressions. The shorter syntax saves time and simplifies the function scope.

What are arrow functions?

An arrow function expression is a more concise syntax for writing function expressions using a "fat arrow" token (=>).

The basic syntax

Below is a basic example of an arrow function:

// ES5 syntax
var multiply = function(x, y) {
  return x * y;
};

// ES6 arrow function
var multiply = (x, y) => { return x * y; };

// Or even simpler
var multiply = (x, y) => x * y;    

You no longer need the function and return keywords, or even the curly brackets.

// everything included
const multiply = function(x, y) => { return x * y };

// remove "function" 
const multiply = (x, y) => { return x * y };

// remove curly brackets and "return" ==> this way it returns implicitly
const multiply = (x, y) => x * y;

// if you only have one argument/parameter 
const multiplyBy2 = x => x * 2;

// if you need to concisely return an object, you can wrap the {} inside the () to avoid syntax conflicts
const getSumProductObject = (x, y) => ({sum : x + y, product: x * y});

// combined with the ternary operator, but note it's not a looker! 
const addOrMultiply = (x, y, mathOperator) => mathOperator.toLowerCase() === 'add' ? x + y : x * y;

A simplified this

Before arrow functions, new functions defined their own this value. To use this inside a traditional function expression, we have to write a workaround like so:

// ES5 syntax
function Person() {
  // we assign `this` to `self` so we can use it later
  var self = this;
  self.age = 0;

  setInterval(function growUp() {
    // `self` refers to the expected object
    self.age++;
  }, 1000);
}

An arrow function doesn't define its own this value, it inherits this from the enclosing function:

// ES6 syntax
function Person(){
  this.age = 0;

  setInterval(() => {
    // `this` now refers to the Person object, brilliant!
    this.age++;
  }, 1000);
}

var p = new Person();

An arrow function does not have its own arguments object. For example, if you do not know the number of arguments passed to a function, instead of using arguments you can use the rest operator:

const myFunc = (...n) => {
  console.log('The first argument is', n[0]);
}
myFunc(10,20,30,40,40); // output: The first argument is 10

Because of this, an arrow function cannot be used as a constructor, hence there's no need for a prototype property.

(() => {}).hasOwnProperty('prototype');  // false

Further Reading

MDN link