JavaScript Closures

What is a closure?

A closure is an inner function that has access to the outer (enclosing) function’s variables—scope chain. The closure has three scope chains: it has access to its own scope (variables defined between its curly brackets), it has access to the outer function’s variables, and it has access to the global variables.

The inner function has access not only to the outer function’s variables, but also to the outer function’s parameters. Note that the inner function cannot call the outer function’s arguments object, however, even though it can call the outer function’s parameters directly.

You create a closure by adding a function inside another function.

A Basic Example of Closures in JavaScript:

function showName (firstName, lastName) {
var nameIntro = "Your name is ";
// this inner function has access to the outer function's variables, including the parameter
function makeFullName () {
return nameIntro + firstName + " " + lastName;
return makeFullName ();
showName ("Michael", "Jackson"); // Your name is Michael Jackson

Closures are used extensively in Node.js; they are workhorses in Node.js’ asynchronous, non-blocking architecture. Closures are also frequently used in jQuery and just about every piece of JavaScript code you read.

A Classic jQuery Example of Closures:

$(function() {
var selections = [];
$(".niners").click(function() { // this closure has access to the selections variable
selections.push (this.prop("name")); // update the selections variable in the outer function's scope

Closures’ Rules and Side Effects

  • Closures have access to the outer function’s variable even after the outer function returns
  • Closures store references to the outer function’s variables
  • Closures Gone Awry

One thought on “JavaScript Closures

  1. bbom

    thank you for being so good at writing and giving information, you do it very well, and your website is very good too, congratulations.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s