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Lasso.js plugin to support compilation of less dependencies


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Lasso.js plugin to support compilation of Less CSS dependencies


npm install lasso-less --save

The lasso-less plugin will then need to be registered as shown below before you can start adding Less dependencies:

require('lasso').configure({ ... plugins: [ 'lasso-less', ... ] });

Configuration can also be passed to the lasso-less plugin if needed:

require('lasso').configure({ ... plugins: [ { plugin: 'lasso-less', config: { /* see below for config options */ } }, ... ] });

Basic Usage


{ "dependencies": [ "./variables.less", "./foo.less", "./bar.less" ] }

The lasso-less plugin will concatenate all of the Less dependencies targeted for the same bundle and pass that as input to the Less renderer. Therefore, given the following contents of each file:


@foo-color: red; @bar-color: green; @logo-image: url(logo.png);


.foo { color: @foo-color; background-image: @logo-image; }


.bar { color: @bar-color; }

The output will be the following:

.foo { color: red; background-image: url(logo-a0db53.png); } .bar { color: green; }

Less Imports

You can use @import (e.g., @import "foo.less";) inside a Less file to import other Less files, but if you want to provide global imports to all Less files across all bundles then you can use the less-import dependency type as shown below:

{ "dependencies": [ "less-import: ./variables.less", "./foo.less", "./bar.less" ] }

The lasso-less plugin also supports resolving Less files using the Node.js module resolver. If you need to include a Less file found in an installed module then you can prefix an import with require: or ~. For example, given the following directory structure:

./ └── node_modules/ └── my-module/ └── foo.less

The foo.less file that is part of the installed my-module can then be added as a dependency using either of the following approaches:

using @import:

@import "require: my-module/foo.less"; // or @import "~my-module/foo.less";

using browser.json:

{ "dependencies": [ "require: my-module/foo.less" ] }


URLs in the form url(<url>) inside Less files will automatically be resolved by this plugin. Unless the URL is an absolute URL, this plugin will attempt to resolve the URL to an actual file on the file system. After resolving the URL to a file, the file will then be sent through the Lasso.js pipeline to produce the final URL.


.foo { background-image: url(foo.png); }


.foo { background-image: url(/static/foo-a0db53.png); }

It is recommended to avoid putting variables inside a url() part. URLs without variables are resolved relative to where the variable is first introduced. URLs with variables are resolved relative to the Less file that makes use of the variable and this will often not be the correct behavior. If you want to change how URLs are resolved, you can provide a custom URL resolver when registering the lasso-less plugin (see next section).

To clarify:

/* Not recommended ☹ */ @logo-image: "logo.png"; .foo { background-image: url("@{logo-image}"); }

Instead, the following is recommended:

/* GOOD! ☺ */ @logo-image: url(logo.png); .foo { background-image: @logo-image; }

Custom URL Resolver

A custom URL resolver can be provided when registering the lasso-less plugin as shown below:

require('lasso').configure({ ... plugins: [ { plugin: 'lasso-less', config: { urlResolver: function(url, context, callback) { if (/^foo:/.test(url)) { callback(null, url.substring(4).toUpperCase()); } else { context.defaultUrlResolver(url, context, callback); } } } }, ... ] });

If provided, all URLs will be resolved using the provided URL resolver. For the default implementation please see: lib/util/default-url-resolver.js

The context argument will contain the following properties:

  • path - The file system path of the containing Less file
  • dir - The parent directory of the containing Less file
  • defaultUrlResolver (Function(url, context, callback)) - The default URL resolver
  • pluginConfig - The configuration passed to the lasso-less plugin when registered
  • lasso - The Lasso.js instance
  • lassoContext - The Lasso.js context object


Example config

require('lasso').configure({ ... plugins: [ { plugin: 'lasso-less', config: { extensions: ['less', 'css'], lessConfig: { // P strictMath: true, strictUnits: true } } } ] });

Configuration properties

  • lessConfig - Passthrough config options to the Less render (object)
  • extensions - An array of file extensions to process as Less files (array, defaults to ['less'])



Last updated on 10 Aug 2021

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