JavaScript functions can be confusing sometimes. There are many ways to define them and many ways to invoke them. Throw in this, and things really starts to get fun.

In this post we will revisit plain-old functions in JavaScript. The different ways we can invoke them, and what this is within the body. We’ll go over the new arrow functions and their differences in behaviour from ordinary functions.

At the end of this post we’ll cover some gotchas when using arrow functions.

Revisiting functions

Let’s review function declarations, how this can be affected by different invocation methods, and the .bind() method.

If you are already familiar with these topics, feel free to jump to the ES6 arrow function section

Function definition

In JavaScript, you can define a function through either a function declaration or a function expression.

function f() {} // declaration
var g = function () {}; // expression and assignment

For the purposes of this post, we won’t go over the differences between the two. (That can be a future topic)

What is this?

The this in a function depends on how it is defined, and how it is invoked.

Take this function for example.

function f(b) {
  return this.a + b;

You can call it in the following ways.

f(1); // window.a + b = undefined + 1 = NaN
f.apply({a: 1}, [2]); // this.a + b = 1 + 2 = 3{a: 1}, 2); // this.a + b = 1 + 2 = 3

If you use a direct function invocation f(1), then this is either window or undefined (in strict mode).

When f.apply() and are used, then this refers to the object passed as the first argument.

Object method

You can also use functions as object methods. In this case, this refers to the object instance.

var obj = { a: 10, f: function(b) { return this.a + b; } };
obj.f(1); // this.a + 1 = 10 + 1 = 11

Yet, this can still be changed using .apply() and .call().

obj.f.apply({a: 1}, [2]); // 3{a: 1}, 2); // 3

Using .bind(…)

Another way we can change the this of a function is through .bind(…), which returns a new function with the same body, but binds this to the new object.

var g = f.bind({a: 100});
g(1); // 101

This this cannot be changed through .apply() and .call().

g.apply({a: 1}, [1]); // 101{a: 1}, 1); // 101

Nor through method call.

obj = { g: g };
obj.g(1); // 101

Arrow functions

ES6 introduces the new arrow function.

var k = (x) => { return x };
var k = x => x; // same thing but shorter

The interesting thing about arrow functions is that this is always bound to the context in which it was defined. This is useful when using it as callbacks when object methods.

class TodosView {
  todos: [],
  initialize() {
    fetch('/todos.json').then((response) => {
      this.todos = response.json();
  render() { /* Do something with this.todos */ }

Because the success arrow function is declared in the scope of initialize(), it takes on the same this, which is of course the TodosView instance.

If this were done with normal function, then this would have been something else (maybe window or undefined). Without the arrow function, we’d have to resort to this.

initialize() {
  var self = this;

  fetch('/todos.json').then(function() {
    self.todos = response.json();

Or this.

initialize() {
  fetch('/todos.json').then((function() {
    this.todos = response.json();

What is this?

Since this is always bound to the context in which the function is closed under, you can never rebind to anything else.

window.who = 'Bob';

var h = () => this.who;

h(); // 'Bob'
h.apply({who: 'Alice'}); // 'Bob'{who: 'Fred'}); // 'Bob'
h.bind({who: 'Mary'})(); // 'Bob'

var obj = { who: 'Sue', h };
obj.h(); // 'Bob'

Never use arrow functions as methods

A corollary of this binding property is that arrow functions should not be used as object methods. This is because this will not refer to the object instance as you would expect it to.

var obj = {
  a: 1,

  // BAD
  f: (b) => {
    return this.a + b;

Again, if you feel like using arrow functions as methods, don’t! Use function short form instead.

var obj = {
  a: 1,

  // GOOD
  f(b) {
    return this.a + b;

The new operator

I left out one more way a function can be invoked: the new operator. The this in a constructor function is the newly created instance.

function F() {
  this.greeting = 'Hello!';

new F().greeting; // 'Hello!'

If you try the same with an arrow function, this is still whatever it was during declaration.

var G = () => {
  this.greeting = 'Hello!';

new G().greeting; // undefined
window.greeting; // 'Hello!'

So if you feel inclined to new an arrow function for some reason, don’t! Use normal functions or classes instead.

function G() {
  this.greeting = 'Hello';

class G {
  constructor() {
    this.greeting = 'Hello';

To sum it up

There are many ways to define and use functions in JavaScript. With ES6 we have even more options open to us. It’s important to know what to use in specific situations, and why. And remember, never use arrow functions as object methods!

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