O'Reilly

JavaScript Functions that Return Functions

By on  

A few weeks back, I tweeted that I loved functions that returned functions. I got quite a few replies to the tune of....WTF?!  It's important that people understand the value of functions that return functions;  using this technique can save you code, JavaScript efficiency, and a gained understanding of how powerful JavaScript can be.  I've created a quick example I'd like to show you so that you can get the idea I was trying to communicate.

Let's say you have one host object with two child objects, both with get methods, and both do exactly the same task but with a different attribute:

var accessors = {
	sortable: {
		get: function() {
			return typeof this.getAttribute('sortable') != 'undefined';
		}
	},
	droppable: {
		get: function() {
			return typeof this.getAttribute('droppable') != 'undefined';
		}
	}
};

Repeating the same code isn't ideal, so we could create one external function, passing it an attribute argument:

function getAttribute(attr) {
	return typeof this.getAttribute(attr) != 'undefined';
}
 
var accessors = {
	sortable: {
		get: function() {
			return getAttribute('sortable');
		}
	},
	droppable: {
		get: function() {
			return getAttribute('droppable');
		}
	}
};

That's a lot better but still not ideal because there's an extra, intermediate function execution every time the method is called.  What would work best is a function that returned the final function  -- that would eliminate the extra function execution with every call to get:

function generateGetMethod(attr) {
	return function() {
		return typeof this.getAttribute(attr) != 'undefined';
	};
}
 
var accessors = {
	sortable: {
		get: generateGetMethod('sortable')
	},
	droppable: {
		get: generateGetMethod('droppable')
	}
};

/* functional equivalent to the original code:

var accessors = {
	sortable: {
		get: function() {
			return typeof this.getAttribute('sortable') != 'undefined';
		}
	},
	droppable: {
		get: function() {
			return typeof this.getAttribute('droppable') != 'undefined';
		}
	}
};

*/

What you see above is a function returning a function; each method gets its own method for getting the property and there's no overhead upon each get call.

This is a really useful technique that saves you from repeating likewise code and, when used correctly, is easy to understand and maintain!

O'Reilly Velocity Conference
Save 20% with discount code AFF20

Recent Features

  • Create a CSS Cube

    CSS cubes really showcase what CSS has become over the years, evolving from simple color and dimension directives to a language capable of creating deep, creative visuals.  Add animation and you've got something really neat.  Unfortunately each CSS cube tutorial I've read is a bit...

  • Vibration API

    Many of the new APIs provided to us by browser vendors are more targeted toward the mobile user than the desktop user.  One of those simple APIs the Vibration API.  The Vibration API allows developers to direct the device, using JavaScript, to vibrate in...

Incredible Demos

  • Animated AJAX Record Deletion Using Dojo

    I'm a huge fan of WordPress' method of individual article deletion. You click the delete link, the menu item animates red, and the item disappears. Here's how to achieve that functionality with Dojo JavaScript. The PHP - Content & Header The following snippet goes at the...

  • Basic AJAX Requests Using MooTools 1.2

    AJAX has become a huge part of the modern web and that wont change in the foreseeable future. MooTools has made AJAX so simple that a rookie developer can get their dynamic pages working in no time. Step 1: The XHTML Here we define two links...

Discussion

  1. I know an old XHR implementation which checked against ActiveX and then returned an XHR handler, specifically for IE or other browsers. Something similar was implemented for event handlers. Modern libraries ought to have this behaviour all over the place. It’s actually a very useful every day technique.

  2. I really love it when you write short little helpful articles like this. Thanks for this technique!

  3. Nice example, with the added partial function as a hidden bonus :) Love JavaScript.

  4. Prince Shahnawaz

    awesome technique.. it might be very useful… ;)

  5. MaxArt

    Guys, this is where Javascript reveals all of its potential.
    Never be afraid to use it. Can save you a lot of code, way more what you can save using jQuery.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!

Recently on David Walsh Blog