Javascript best practices

Sunday, July 10, 2016

Where to put the javascript?

Unless it’s essential for it to be in the head of the HTML document, place all javascript just before the closing body tag of a page.

What’s essential for the head?

That can vary from project to project but a rule of thumb would be:

  • A minimal amount of inlined javascript
  • Any javascript that can be loaded async.

What should do in the footer

  • jQuery. Nothing essential should be using jQuery.
  • Everything else like galleries and slider js, social js etc.

How many js files should there be?

For a normal site there should be 2 js files that are loaded for the whole site. The header.js and the footer.js. That’s it! Call them what you like! There may be more specific files that are loaded for certain pages or sections of a site.


Contains the header javascript that contains things like Modernizr. This can be created by concatenating (joining together) lots of smaller js files.


Contains everything else that is global js. Concatenated from other js files like jQuery/Zepto and other plugins and libraries. They should be conactenated in a logical order, starting with the biggest libraries, then plugins and then the callbacks to activate the plugins.

Writing style


Things that can change once the code is running e.g. counters. These should be lowercase with underscores.

1var variable_things = 123; ### Constants 

Are variables too but they are ones that stay the same e.g. A DOM element that won’t change.

1var CONSTANT_THING = ‘I never change! I am a rock.’; 

If you find yourself doing something late in your code like CONSTANT_THING = 'Something else'; then it’s not a constant and should be refactored to lowercase.

Line endings

Should finish with a semi-colon. Older browsers will fall over if you don’t do this. These are not really required by modern browsers, but make things clearer and let’s do it right anyway. The browser engine should not have to assume.

1var me = 'Carl'; var you = 'Not Carl (Probably)'; 

Writing efficient code

If you have to write something more than once it should be a variable or a function. This makes your code clear and maintainable. It also saves the browser from having to repeat the same actions each time you use jQuery to select the same DOM elements.


Stay away from:

1$('.thing').on('click' function() { ... }); $('.thing').on('tap' function() { ... }); $('.thing').on('swipe' function() { ... }); 

Try to do:

1var THING = $('.thing'); THING.on('click' function() { ... }); THING.on('tap' function() { ... }); THING.on('swipe' function() { ... }); 

The second example looks more code, but imagine that this both examples are spread over a massive plugin you just wrote. Now I know this never happens but what if you happen to change the class of .thing to .other-thing. Reduce the possibility of missing one instance of your variable, and of making a typo. You’re lining yourself up for bugs and broken plugin. Choose the option that is easier to maintain.


Stay away from:

Don’t do similar things on multiple functions. It’s also more to maintain and more to go wrong or debug.

1$('.next').on('click', function(e) { current_slide++; if (current_slide > MAX_SLIDES) { current_slide = 1; } //Logic to update slides... e.preventDefault(); //stops default action of clicking things. }); $('.next').on('swipe-right', function(e) { current_slide++; if (current_slide > MAX_SLIDES) { current_slide = 1; } //Same logic to update slides... e.preventDefault(); //stops default action of clicking things. }); $('.index').on('click', function(e) { current_slide++; if (current_slide > MAX_SLIDES) { current_slide = 1; } //Same logic to update slides... e.preventDefault(); //stops default action of clicking things. }); 

Try to do:

You can see from this example that the slide update functionality is in only one place. This means it’s easy to manage and fix when things go wrong and when you look at other functions the logic should be easy to follow.

1function update_slides() { //Logic to update slides... //Logic to update index can be in the same place to keep it neat or it can reference it's own unique function too! } function next_slide() { current_slide++; if (current_slide > MAX_SLIDES) { current_slide = 1; } update_slides(); } function prev_slide() { current_slide--; if (current_slide < 1) { current_slide = MAX_SLIDES; } update_slides(); } function goto_slide(slide_num) { //Constrain the slide within the range available. if (current_slide < 1) { current_slide = MAX_SLIDES; } else if (current_slide > MAX_SLIDES) { current_slide = 1; } update_slides(); } //Somewhere else in the app... $('.next').on('click', function(e) { next_slide(); e.preventDefault(); //stops default action of clicking things. }); 

How to write a JavaScript class

A class is a code template for creating objects that provides initial values and functions and can be extended to create new classes. There is more than one way to make a class in javascript. Best stick to one, as follows. Note the CamelCase Style for the class name.

1/* @class Playlist */ // function Playlist(name, tracks) { = name || 'my playlist'; this.tracks = tracks || []; } //By using prototype we don't have to duplicate core object functions //requests for functions and properties are passed up the protostupe chain //meaning less duplication of code Playlist.prototype = {}; //Add functionallity with functions for the class Playlist.prototype.addTrack = function(track) { this.tracks.push(track); }; Playlist.prototype.getTrackList = function(format) { if (typeof format !== 'undefined' ) { return this.tracks.join(format); } else { return this.tracks; } }; /* End @class Playlist */ //Now we have the class we can use it //make a new Playlist object using the Playlist class var my_playlist = new Playlist('Such Wow', 99); //add a track to the list my_playlist.addTrack(1234567890); //print the tracklist to the console console.log( my_playlist.getTrackList() ); 

Read more about prototypes and why they are useful and get your head around the concept.

Validating code

To make sure your code is nice, use a validator. is usually good enough, but if you want a challenge go with

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