How to concatenate and minify multiple CSS and JavaScript files with Grunt.js (0.3.x)

How to concatenate and minify multiple CSS and JavaScript files with Grunt.js (0.3.x)

Note: This question is only relevant for Grunt 0.3.x and has been left for reference. For help with the latest Grunt 1.x release please see my comment below this question.
I’m currently trying to use Grunt.js to setup an automatic build process for first concatenating and then minifying CSS and JavaScript files.
I have been able to successfully concatenate and minify my JavaScript files, although each time I run grunt it seems to just append to the file instead of overwriting them.
As for the minifying or even concatenating CSS, I have been unable to do this as of yet!
In terms of grunt CSS modules I have tried using consolidate-css, grunt-css & cssmin but to no avail. Could not get my head around how to use them!
My directory structure is as follows (being a typical node.js application):

/public/css/[various css files]
/public/js/[various javascript files]

Here is what my grunt.js file currently looks like in the root folder of my application:
module.exports = function(grunt) {

// Project configuration.
pkg: ‘‘,
concat: {
dist: {
src: ‘public/js/*.js’,
dest: ‘public/js/concat.js’
min: {
dist: {
src: ‘public/js/concat.js’,
dest: ‘public/js/concat.min.js’
jshint: {
options: {
curly: true,
eqeqeq: true,
immed: true,
latedef: true,
newcap: true,
noarg: true,
sub: true,
undef: true,
boss: true,
eqnull: true,
node: true
globals: {
exports: true,
module: false
uglify: {}

// Default task.
grunt.registerTask(‘default’, ‘concat min’);


So just to summarise I need help with two questions:

How to concatenate and minify all my css files under the folder /public/css/ into one file, say main.min.css
Why does grunt.js keep on appending to the concatenated and minified javascript files concat.js and concat.min.js under /public/js/ instead of overwriting them each time the command grunt is run?

Updated 5th of July 2016 – Upgrading from Grunt 0.3.x to Grunt 0.4.x or 1.x
Grunt.js has moved to a new structure in Grunt 0.4.x (the file is now called Gruntfile.js). Please see my open source project Grunt.js Skeleton for help with setting up a build process for Grunt 1.x.
Moving from Grunt 0.4.x to Grunt 1.x should not introduce many major changes.


Solution 1:

concat.js is being included in the concat task’s source files public/js/*.js. You could have a task that removes concat.js (if the file exists) before concatenating again, pass an array to explicitly define which files you want to concatenate and their order, or change the structure of your project.

If doing the latter, you could put all your sources under ./src and your built files under ./dest

├── css
│   ├── 1.css
│   ├── 2.css
│   └── 3.css
└── js
    ├── 1.js
    ├── 2.js
    └── 3.js

Then set up your concat task

concat: {
  js: {
    src: 'src/js/*.js',
    dest: 'dest/js/concat.js'
  css: {
    src: 'src/css/*.css',
    dest: 'dest/css/concat.css'

Your min task

min: {
  js: {
    src: 'dest/js/concat.js',
    dest: 'dest/js/concat.min.js'

The build-in min task uses UglifyJS, so you need a replacement. I found grunt-css to be pretty good. After installing it, load it into your grunt file


And then set it up

cssmin: {
    src: 'dest/css/concat.css',
    dest: 'dest/css/concat.min.css'

Notice that the usage is similar to the built-in min.

Change your default task to

grunt.registerTask('default', 'concat min cssmin');

Now, running grunt will produce the results you want.

├── css
│   ├── concat.css
│   └── concat.min.css
└── js
    ├── concat.js
    └── concat.min.js

Solution 2:

I want to mention here a very, VERY, interesting technique that is being used in huge projects like jQuery and Modernizr for concatenate things.

Both of this projects are entirely developed with requirejs modules (you can see that in their github repos) and then they use the requirejs optimizer as a very smart concatenator. The interesting thing is that, as you can see, nor jQuery neither Modernizr needs on requirejs to work, and this happen because they erase the requirejs syntatic ritual in order to get rid of requirejs in their code. So they end up with a standalone library that was developed with requirejs modules! Thanks to this they are able to perform cutsom builds of their libraries, among other advantages.

For all those interested in concatenation with the requirejs optimizer, check out this post

Also there is a small tool that abstracts all the boilerplate of the process: AlbanilJS

Solution 3:

I agree with above answer. But here is another way of CSS compression.

You can concat your CSS by using YUI compressor:

module.exports = function(grunt) {
  var exec = require('child_process').exec;
   grunt.registerTask('cssmin', function() {
    var cmd = 'java -jar -Xss2048k '
      + __dirname + '/../yuicompressor-2.4.7.jar --type css '
      + grunt.template.process('/css/style.css') + ' -o '
      + grunt.template.process('/css/style.min.css')
    exec(cmd, function(err, stdout, stderr) {
      if(err) throw err;

Solution 4:

You don’t need to add the concat package, you can do this via cssmin like this:

cssmin : {
      options: {
            keepSpecialComments: 0
      minify : {
            expand : true,
            cwd : '/library/css',
            src : ['*.css', '!*.min.css'],
            dest : '/library/css',
            ext : '.min.css'
      combine : {
        files: {
            '/library/css/app.combined.min.css': ['/library/css/main.min.css', '/library/css/font-awesome.min.css']

And for js, use uglify like this:

uglify: {
      my_target: {
        files: {
            '/library/js/app.combined.min.js' : ['/app.js', '/controllers/*.js']

Solution 5:

I think may be more automatic, grunt task usemin take care to do all this jobs for you, only need some configuration: