Compile LESS using Grunt

The topic describes how to install, configure, and use Grunt JavaScript task runner to compile .less files in Magento 2.

Prerequisites

• Make sure that you set your Magento application to the developer or default mode. The default mode sets the Less compilation mode to Server-side Less compilation.
• Install and configure Grunt as described in Using Grunt for Magento tasks

Adding themes to Grunt configuration

To compile .less files, add your theme to module.exports in the Grunt configuration, either in the default dev/tools/grunt/configs/themes.js or in the custom configuration file. For example:

1. Install node.js to any location on your machine.

2. Install the Grunt CLI globally. To do this, run the following command in a command prompt:

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   npm install -g grunt-cli
   

3. From the <Magento_root> directory, copy and paste the contents of the following files:

   • package.json.sample to package.json
   • Gruntfile.js.sample to Gruntfile.js
   • grunt-config.json.sample into grunt-config.json

4. Install (or refresh) the node.js project dependency, including Grunt, for your Magento instance. To do this, run the following commands in a command prompt:

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   cd <your_Magento_instance_directory>
   

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   npm install
   

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   npm update
   

5. Add your theme to Grunt configuration. To do this, in the dev/tools/grunt/configs/themes.js file, add your theme to module.exports like following:

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   <theme>: {
       area: '<area>',
       name: '<Vendor>/<theme>',
       locale: '<language>',
       files: [
           '<path_to_file1>', //path to root source file
           '<path_to_file2>'
       ],
       dsl: 'less'
   }
   

   Where the following notation is used:

   • <Vendor>: vendor name.
   • <theme>: your theme code, conventionally should correspond to the theme directory name.
   • <language>: specified in the code_subtag format, for example en_US. Only one locale can be specified here. To debug the theme with another locale, create one more theme declaration, having specified another value for language.
   • <area>: area code, can be either frontend or adminhtml.
   • <path_to_file>: path to the root source file, relative to the app/design/frontend/<Vendor>/<theme>/web directory. You need to specify all root source files of the theme. If your theme inherits from a certain theme, and does not contain its own root source files, specify the root source files of the parent theme.

   IMPORTANT NOTE : If grunt gives the error “Error: Cannot find module ….”, check the path in your grunt-config.json file and correct as necessary.

6. (Optional) If you want to use Grunt for “watching” changes automatically, without reloading pages in a browser each time, install the LiveReload extension in your browser.

Grunt commands

The following table describes the grunt commands you can use to perform different customization tasks. Run all commands from your Magento installation directory.

Use cases of tracking changes using Grunt

The following shows which Grunt tasks to use for debugging:

• After you switch the compilation mode from client-side to server-side, run the exec command.
• After you customize the content of any .less file, except the root source files, run the less task and reload the page.
• After you customize the root source files or move the files included to the root files, run the exec command and reload the page.
• After you run php bin/magento setup:upgrade, run exec command.
• After you run the exec command, run the clear command to clear the Magento cache, then run the watch command. Running the commands in this order will ensure that any custom jQuery attributes like product sliders, banners, etc are loaded correctly.

If you have LiveReload installed, run the grunt watch command, and the flow is even simpler:

• After you customize the content of any .less file, changes are applied and the page reloads automatically. No additional changes
• After you customize the root source files or move the files included to the root files, run the clean and exec commands, which reloads the page in the browser.

CSS source maps

When using Grunt for styles preprocessing, you can enable the CSS source maps generation in your browser. It will make the theme styles debugging easier.

For each theme, Magento compiles all theme .less files into two CSS files: styles-m.css and styles-l.css. So when you debug a theme, your browser only sees styles-m.css and it might be difficult to define which .css or .less file requires corrections. For example:

CSS source maps solve this issue. They help to find the .less file, where the style is specified. For example:

CSS source maps are generated automatically when you compile CSS for your theme using the grunt less: <theme> command. To use them, you need to enable the display of source maps in your browser. For example, in Chrome, you would open the Developer Tools, go to the Settings panel, select Preferences, then check the Enable CSS source maps checkbox.

Magento has a base set of variables that define commonly used aspects of a theme; such as colors, fonts, style of page titles, and so on.

The <magento-root>/lib/web/css/source/lib/variables directory contains LESS files that define values assigned to variables for many of the common elements in Magento.

To change or override any of these variables, simply create a file in <theme-dir>/web/css/source/_theme.less For example:

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@navigation__background: @secondary__color__light;
@font-family__sans-serif: 'Helvetica Neue', Helvetica, Arial, sans-serif;