Less is a CSS pre-processor, meaning that it extends the CSS language, adding features that allow variables, mixins, functions and many other techniques that allow you to make CSS that is more maintainable, themable and extendable.

Less runs inside Node, in the browser and inside Rhino. There are also many 3rd party tools that allow you to compile your files and watch for changes.

For example:

@base: #f938ab;

.box-shadow(@style, @c) when (iscolor(@c)) {
  -webkit-box-shadow: @style @c;
  box-shadow:         @style @c;
}
.box-shadow(@style, @alpha: 50%) when (isnumber(@alpha)) {
  .box-shadow(@style, rgba(0, 0, 0, @alpha));
}
.box {
  color: saturate(@base, 5%);
  border-color: lighten(@base, 30%);
  div { .box-shadow(0 0 5px, 30%) }
}

compiles to

.box {
  color: #fe33ac;
  border-color: #fdcdea;
}
.box div {
  -webkit-box-shadow: 0 0 5px rgba(0, 0, 0, 0.3);
  box-shadow: 0 0 5px rgba(0, 0, 0, 0.3);
}

Less can be used on the command line via npm, downloaded as a script file for the browser or used in a wide variety of third party tools. See the Usage section for more detailed information.

Installation

The easiest way to install Less on the server, is via npm, the node.js package manager, as so:

$ npm install -g less

Command-line usage

Once installed, you can invoke the compiler from the command-line, as such:

$ lessc styles.less

This will output the compiled CSS to stdout, you may then redirect it to a file of your choice:

$ lessc styles.less > styles.css

To output minified CSS, simply pass the -x option. If you would like more involved minification, the Clean CSS is also available with the --clean-css option.

To see all the command line options run lessc without parameters.

Usage in Code

You can invoke the compiler from node, as such:

var less = require('less');

less.render('.class { width: (1 + 1) }', function (e, css) {
  console.log(css);
});

which will output

.class {
  width: 2;
}

you may also manually invoke the parser and compiler:

var parser = new(less.Parser);

parser.parse('.class { width: (1 + 1) }', function (err, tree) {
  if (err) {
    return console.error(err)
  }
  console.log(tree.toCSS());
});

Configuration

You may pass some options to the compiler:

var parser = new(less.Parser)({
  paths: ['.', './lib'], // Specify search paths for @import directives
  filename: 'style.less' // Specify a filename, for better error messages
});

parser.parse('.class { width: (1 + 1) }', function (e, tree) {
  tree.toCSS({
    // Minify CSS output
    compress: true
  });
});

Third party tools

See the Usage section for details of other tools.

Command-line With Rhino

Each less.js release contains also rhino-compatible version.

Command line rhino version requires two files:

  • less-rhino-.js - compiler implementation,
  • lessc-rhino-.js - command line support.

Command to run the compiler:

java -jar js.jar -f less-rhino-<version>.js lessc-rhino-<version>.js styles.less styles.css

This will compile styles.less file and save the result to styles.css file. The output file parameter is optional. If it is missing, less will output the result to stdout.

Client-side usage

Using less.js in the browser is great for development, but it's not recommended for production

Client-side is the easiest way to get started and good for developing with Less, but in production, when performance and reliability is important, we recommend pre-compiling using node.js or one of the many third party tools available.

To start off, link your .less stylesheets with the rel attribute set to "stylesheet/less":

<link rel="stylesheet/less" type="text/css" href="styles.less" />

Next, download less.js and include it in a <script></script> tag in the <head> element of your page:

<script src="less.js" type="text/javascript"></script>

Tips

  • Make sure you include your stylesheets before the script.
  • When you link more than one .less stylesheet each of them is compiled independently. So any variables, mixins or namespaces you define in a stylesheet are not accessible in any other.

Browser Options

Options are defined by setting them on a global less object before the <script src="less.js"></script>:

<!-- set options before less.js script -->
<script>
  less = {
    env: "development",
    async: false,
    fileAsync: false,
    poll: 1000,
    functions: {},
    dumpLineNumbers: "comments",
    relativeUrls: false,
    rootpath: ":/a.com/"
  };
</script>
<script src="less.js"></script>

Learn more about Browser Options

Browser downloads

Download Less.js v1.7.4

Download source code

Get the latest Less.js source code by downloading it directly from GitHub.

Clone or fork via GitHub

Fork the project and send us a pull request!

Install with Bower

Install the Less.js project and JavaScript by running the following in the command line:

bower install less

Less CDN

<script src="//cdnjs.cloudflare.com/ajax/libs/less.js/1.7.4/less.min.js"></script>

Less.js is released under the Apache 2 License (though there are plans to dual license it). Copyright 2009-2014, Alexis Sellier and the Less Core Team (see about). Boiled down to smaller chunks, it can be described with the following conditions.

It allows you to:

  • Freely download and use Less.js, in whole or in part, for personal, company internal or commercial purposes
  • Use Less.js in packages or distributions that you create

It forbids you to:

  • Redistribute any piece of Less.js without proper attribution

It requires you to:

  • Include a copy of the license in any redistribution you may make that includes Less.js
  • Provide clear attribution to The Less Core Team for any distributions that include Less.js

It does not require you to:

  • Include the source of Less.js itself, or of any modifications you may have made to it, in any redistribution you may assemble that includes it
  • Submit changes that you make to Less.js back to the Less.js project (though such feedback is encouraged)

The full Less.js license is located in the project repository for more information.