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Public vs. private functions in JavaScript

Today, reader Kevin Marmet asked (shared with permission):

I’m trying to understand #javascript patterns — especially private and public functions.

Let’s dig into this a bit.

Public vs. Private Functions

So what’s the difference between a public and private function?

A private function can only be used inside of it’s parent function or module. A public function can be used inside or outside of it. Public functions can call private functions inside them, however, since they typically share the same scope.

Providing public access to some functions but not others is helpful when building plugins and other modular scripts.

An example

For example, in Houdini, an accordion script I wrote, users initialize the plugin like this.


The init() method is a public function.

While the script automatically opens and closes accordion content when users click toggle links, I also provide developers with the ability to dynamically open or close content from their own scripts using some additional public methods.

// Shows the accordion content with the ID `#some-content`

// Hides the accordion content with the ID `#some-other-content`

In this example, openContent() and closeContent() are also public methods.

Houdini also includes some private methods that are used within the plugin, but can’t be access by developers. For example, I use a helper function to bring newly opened content into focus for visitors using assistive technology like screen readers.

var adjustFocus = function ( content, settings ) {
	// Do stuff...

This function cannot be called from another developer’s script. It’s private.

How to do this with your own scripts

The secret sauce that makes this all work is a JavaScript pattern known as the Revealing Module Pattern.

Here’s an example of a plugin called beNice() that you can use to say nice things.

The smile() method is public. You can pass in a message as an argument, or let it create one for you. It uses the private saySomethingNice() method to alert() your message.

var beNice = (function () {

	'use strict';

	// My public methods will get added to this object
	var publicAPIs = {};

	// A private method
	var saySomethingNice = function (somethingNice) {

	// A public method = function (message) {
		if (message) {
		} else {
			saySomethingNice('You make the world better just by being you!');

	// Return our public methods so that they can be accessed
	return publicAPIs;


If you call (with or without a message passed in), it will show an alert with a nice message. If you try to call saySomethingNice(), you’ll get an error.

Uncaught ReferenceError: saySomethingNice is not defined