Gulpfile.js

var gulp = require('gulp');
var sourcemaps = require('gulp-sourcemaps');
var source = require('vinyl-source-stream');
var buffer = require('vinyl-buffer');
var browserify = require('browserify');
var watchify = require('watchify');
var babel = require('babelify');

function compile(watch, file) {
    var bundler = watchify(browserify('./src/inject/'+ file +'.js', { debug: true }).transform(babel));

    function rebundle() {
        bundler.bundle()
            .on('error', function(err) { console.error(err); this.emit('end'); })
            .pipe(source(file + '.min.js'))
            .pipe(buffer())
            .pipe(sourcemaps.init({ loadMaps: true }))
            .pipe(sourcemaps.write('./'))
            .pipe(gulp.dest('./build'));
    }

    if (watch) {
        bundler.on('update', function() {
            console.log('-> bundling...');
            rebundle();
        });
    }

    rebundle();
}

function watch(file) {
    return compile(true, file);
};

gulp.task('build-inject', function() { return compile(false, "inject"); });
gulp.task('watch-inject', function() { return watch("inject"); });

gulp.task('default', ['watch-inject']);