How to configure sourceMaps for LESS using Grunt?

Question

I'm using grunt 0.4.2 and grunt-contrib-less 0.9.0. I want my LESS to be compiled into CSS with support for source maps.

My LESS files are in public/less, and the main one is called main.less.

The compiling of public/less/main.less into public/css/main.css works, but source maps don't work.

What is wrong with my Grunt config below?

{
    less: {
        dev: {
            options: {
                compress: true,
                yuicompress: true,
                optimization: 2,
                sourceMap: true,
                sourceMapFilename: "public/css/main.css.source-map.json", //Write the source map to a separate file with the given filename.
                sourceMapBasepath: "public/less", //Sets the base path for the Less file paths in the source map.
                sourceMapRootpath: "/"//Adds this path onto the Less file paths in the source map.
            },
            files: {
                "public/css/main.css": "public/less/main.less"
            }
        }
    },
    watch: {
        styles: {
            files: ["public/less/*"],
            tasks: ['less'],
            options: {
                livereload: true,
                nospaces: true
            }
        }
    }
}

I don't want to have my CSS created in my /public/less folder; I want to put it into /public/css. Otherwise, I could use this other config, which works:

{
    less: {
        dev: {
            options: {
                compress: true,
                yuicompress: true,
                optimization: 2,
                sourceMap: true,
                sourceMapFilename: "public/less/main.css.map", //I DO NOT WANT THE CSS MAP HERE
                sourceMapBasepath: "public/less", //Sets the base path for the Less file paths in the source map.
            },
            files: {
                "public/less/main.css": "public/less/main.less"//I DO NOT WANT THE CSS HERE
            }
        }
    },
    watch: {
        styles: {
            files: ["public/less/*"],
            tasks: ['less'],
            options: {
                livereload: true,
                nospaces: true
            }
        }
    }
}

Answer 1

If you're still having trouble with it try setting the SourceMapURL to the full path, for example:

http://www.yourdomain.com/assets/css/styles.css.map

Obviously this is a bit of a pain as it's not relative, so when you move your site it will need to be changed. I had the same issue as you and this got it working for me.

Another point I discovered is that you cannot load the SourceMaps from the file system. It has to be from a web server. To get around the file system issue I believe you can inline the SourceMap.

This thread on GitHub has a lot of information.

https://github.com/less/less.js/issues/1050#issuecomment-25621390

Answer 2

I found the LESS site documentation to be more clear regarding params used by grunt-contrib-less.

LESS: Command Line Usage http://lesscss.org/usage/#command-line-usage-installing-lessc

NPM: grunt-contrib-less https://www.npmjs.org/package/grunt-contrib-less

File structure:

laravel/gruntfile.js
laravel/public/less/main.less
laravel/public/css/main.css
laravel/public/css/main.css.map

File 'main.css.map' note:

• If you wish, you can rename to: main.css.source-map.json
• I guess you have some server rule setup that doesn't server *.map files properly from the 'css' folder

Compression notes:

• cleancss: true = will remove sourceMappingURL comment from main.css
• yuicompress: true = will NOT remove sourceMappingURL comment from main.css

Gruntfile.js

less: {
    dev: {
        options: {
            compress: true,
            yuicompress: true,
            optimization: 2,
            sourceMap: true,
            sourceMapFilename: 'public/css/main.css.map', // where file is generated and located
            sourceMapURL: '/css/main.css.map', // the complete url and filename put in the compiled css file
            sourceMapBasepath: 'public', // Sets sourcemap base path, defaults to current working directory.
            sourceMapRootpath: '/', // adds this path onto the sourcemap filename and less file paths
        },
        files: {
            'public/css/main.css': 'public/less/main.less',
        }
    }
},

watch: {
    styles: {
        files: ["public/less/*"],
        tasks: ['less'],
        options: {
            livereload: true,
            nospaces: true
        }
    }
},

laravel/public/css/main.css

.class{ compiled css here } /*# sourceMappingURL=/css/main.css.map */

laravel/public/css/main.css.map

{"version":3,"sources":["/less/main.less"], etc...