Single gulpfile.js to compile SASS and minify JS on multiples folders on a WordPress project.

examples.js

gulp // Compile all files from all folders
gulp css // Compile all SASS files from all folders
gulp js // Minify all JS files from all folders

gulp --plugin1 // Compile all files from plugin1 folder
gulp css --plugin1 // Compile all SASS files from plugin1 folder
gulp js --plugin1 // Minify all JS files from plugin1 folder

gulp --plugin1 --theme1 // Compile all files from plugin1 and theme1 folders
gulp css --plugin1 --theme1 // Compile all SASS files from plugin1 and theme1 folders
gulp js --plugin1 --theme1 // Minify all JS files from plugin1 and theme1 folders

gulp watch // Run default function and start to watch for file changes
gulp gulpfolders // Look into all folders for a gulp.me file and add its parent to gulpfolders.json

// Where "plugin1" and "theme1" are names defined on gulpfolders.json to refer a path

gulpfile.js

/**
 * REQUIRES
 */
var	args         = require('yargs').argv
	autoPrefixer = require('gulp-autoprefixer'),
	cleanCSS     = require('gulp-clean-css'),
	combiner     = require('stream-combiner2'),
	del          = require('del'),
	es           = require('event-stream'),
	fs           = require('fs'),
	glob         = require('glob'),
	gulp         = require('gulp'),
	print        = require('gulp-print'),
	rename       = require('gulp-rename'),
	sass         = require('gulp-sass'),
	sourceMaps   = require('gulp-sourcemaps'),
	uglify       = require('gulp-uglify'),
	folders      = require('./gulpfolders.json')

/**
 * VARIABLES
 */
var base = 'wp-content/',
	paths = {
		sass: '/assets/sass/*.scss',
		css: '/assets/css',
		sourceJS: '/assets/js/source/*.js',
		distJS: '/assets/js'
	}

/**
 * FUNCTIONS
 */
function generateGulpFoldersJSON() {
	var folders = {}
	glob.sync( base + '**/gulp.me').map( folder => {
		var folder = folder.replace( '/gulp.me', ''),
			pathParts = folder.split('/')

		folders[pathParts[pathParts.length - 1]] = folder

		return true
	})

	json = JSON.stringify( folders )
	fs.writeFileSync( 'gulpfolders.json', json, 'utf8' )
}

function getFoldersToWorkOn() {
	var foldersToWorkOn = Object.keys( folders ).filter( key => {
		if ( args[key] ) {
			return true
		}
	})

	if ( 0 === foldersToWorkOn.length ) {
		foldersToWorkOn = Object.keys( folders )
	}

	return foldersToWorkOn
}

function getFolderKeyFromPath( path ) {
	var folderKey = null

	Object.keys( folders ).forEach( key => {
		if ( 0 < path.indexOf( folders[key] ) ) {
			folderKey = key
		}
	})

	return folderKey
}

function compileCSS( folderKey ) {
	del.sync( [folders[folderKey] + paths.css + '/*.css'] )

	var combined = combiner.obj([
		gulp.src( folders[folderKey] + paths.sass ),
			print(),
			sourceMaps.init(),
			sass(),
			autoPrefixer(),
			gulp.dest( folders[folderKey] + paths.css ),
			cleanCSS(),
			rename({ suffix: '.min' }),
			sourceMaps.write(),
		gulp.dest( folders[folderKey] + paths.css )
	])
	combined.on('error', console.error.bind(console))

	return combined
}

function minifyJS( folderKey ) {
	del.sync( [folders[folderKey] + paths.distJS + '/*.js'] )

	var combined = combiner.obj([
		gulp.src( folders[folderKey] + paths.sourceJS ),
			print(),
			sourceMaps.init(),
			uglify(),
			rename({ suffix: '.min' }),
			sourceMaps.write(),
		gulp.dest( folders[folderKey] + paths.distJS )
	])
	combined.on('error', console.error.bind(console))

	return combined
}

/**
 * GULP TASKS
 */
gulp.task( 'gulpfolders', function() {
	return generateGulpFoldersJSON()
})

gulp.task( 'css', function() {
	var streams = getFoldersToWorkOn().map( compileCSS )
	return es.merge( streams )
})

gulp.task( 'js', function() {
	var streams = getFoldersToWorkOn().map( minifyJS )
	return es.merge( streams )
})

gulp.task( 'default', [ 'css', 'js' ] );

gulp.task('watch', ['default'], function() {
	gulp.watch(
		Object.keys( folders ).map( folderKey => {
			return folders[folderKey] + paths.sass.replace( '*.scss', '**/*.scss' )
		})
	).on('change', function(event) {
		console.log('File ' + event.path + ' was ' + event.type + ', running tasks...')
		args = []
		args[getFolderKeyFromPath( event.path )] = true
		gulp.start( 'css' )
	});

	gulp.watch(
		Object.keys( folders ).map( folderKey => {
			return folders[folderKey] + paths.sourceJS
		})
	).on('change', function(event) {
		console.log('File ' + event.path + ' was ' + event.type + ', running tasks...')
		args = []
		args[getFolderKeyFromPath( event.path )] = true
		gulp.start( 'js' )
	});
});

gulpfolders.json

{}

package.json

{
  "name": "gulp-sass-js-multiples-folders",
  "version": "1.0.0",
  "description": "<!DOCTYPE html> <html> <head> \t<meta name=\"viewport\" content=\"width=device-width\" /> \t<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" /> \t<title>WordPress &#8250; ReadMe</title> \t<link rel=\"stylesheet\" href=\"wp-admin/css/install.css?ver=20100228\" type=\"text/css\" /> </head> <body> <h1 id=\"logo\"> \t<a href=\"https://wordpress.org/\"><img alt=\"WordPress\" src=\"wp-admin/images/wordpress-logo.png\" /></a> \t<br /> Version 4.7 </h1> <p style=\"text-align: center\">Semantic Personal Publishing Platform</p>",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "del": "^2.2.2",
    "event-stream": "^3.3.4",
    "fs": "0.0.1-security",
    "glob": "^7.1.1",
    "gulp": "^3.9.1",
    "gulp-autoprefixer": "^3.1.1",
    "gulp-clean-css": "^3.0.4",
    "gulp-print": "^2.0.1",
    "gulp-rename": "^1.2.2",
    "gulp-sass": "^3.1.0",
    "gulp-sourcemaps": "^2.5.1",
    "gulp-uglify": "^2.1.2",
    "stream-combiner2": "^1.1.1",
    "yargs": "^7.0.2"
  }
}