LESS - Generate a deep list of all a file's @imports

gulp-task.js

var gulp = require('gulp'),
	watch = require('gulp-watch'),
	getImports = require('./lessImportList');

gulp.task('less', function(done) {
	getImports(['less/themes/default/app.less'], function(imports) {
		watch(imports)
			.pipe(/* Task logic here */)
	});
});

gulp-watch-less.js

var through = require('through2'),
    fs = require('fs'),
    gulp = require('gulp'),
    gutil = require('gulp-util'),
    watch = require('gulp-watch'),
    gulpLess = require('gulp-less'),
    less = require('less'),
    PluginError = gutil.PluginError;

const PLUGIN_NAME = 'gulp-watch-less';

function getImportsList(file, options, cb) {
    // Parse options
    if (!options) options = {};
    options.filename = file.path;

    // Create new parser instance
    var parser = new less.Parser(options);

    // Parse the source into AST tree via LESS
    parser.parse(file.contents.toString('utf8'), function (err, tree) {
        if (err) { throw new PluginError(PLUGIN_NAME, err); } 

        // Return array of all `@import`ed files via the parsers `imports` object!
        cb(Object.keys(parser.imports.files));
    });
}

function gulpWatchLess(glob, options, cb) {
    var watchFiles = [],
        watchStream;

    // Handles (glob, cb) signature
    if (typeof options === 'function') {
        cb = options;
        options = {};
    }
    if (!options) options = {};

    // Setup watch steam on the provided globs
    watchStream = watch(glob, options, cb);

    // Grab reference to all files in the provided glob via `gulp-src`
    gulp.src(glob).pipe(
        // Create a pass-through stream
        through.obj(function(file, enc, cb) {
            // Null files passthrough
            if(file.isNull()) { return cb(); }

            // Don't support streams
            else if(file.isStream()) {
                throw new PluginError(PLUGIN_NAME, 'Streams not supported!');
            }

            // Grab the list of imports, passing throuh less options if defined
            getImportsList(file, options.less, function(imports) {
                // Push returned imports list onto the watched files array
                watchFiles = watchFiles.concat(imports);

                cb();
            });

            // Passthrough file
            this.push(file);
        },

        // On stream end, watch the files and pipe to output stream
        function(cb) { watch(watchFiles).pipe(watchStream); })
    );

    return watchStream;
}

module.exports = gulpWatchLess;

lessImportList.js

var fs = require('fs'),
	less = require('less');

module.exports = function(path, opts, cb) {
	// Support (path, cb) signature
	if(typeof opts === 'function') {
		cb = opts; opts = null;
	}

	// Read the file (async)
	fs.readFile(path, function(err, source) {
		if(err) { throw new Error(err); }

		// Setup options
		opts || (opts = {});
		opts.filename = path;

		// Create new parser instance
		parser = new less.Parser(opts);

		// Parse the source into AST tree via LESS
		parser.parse(source.toString('utf8'), function (err, tree) {
			if (err) { throw new Error(err); } 

			// Return array of all `@import`ed files via the parsers `imports` object!
			cb(Object.keys(parser.imports.files));
		});
	});
};

usage.js

var getImports = require('./lessImportList');

// Sample usage - log all @import paths to the console :)
getImports('less/themes/default/app.less', function(imports) {
	console.log(imports); // Log to the console
});