There's an official recipe for this: Generating a file per folder

var fs = require('fs');

var path = require('path');

var merge = require('merge-stream');

var gulp = require('gulp');

var concat = require('gulp-concat');

var rename = require('gulp-rename');

var uglify = require('gulp-uglify');



var scriptsPath = 'src/scripts';



function getFolders(dir) {

    return fs.readdirSync(dir)

      .filter(function(file) {

        return fs.statSync(path.join(dir, file)).isDirectory();

      });

}



gulp.task('scripts', function() {

   var folders = getFolders(scriptsPath);



   var tasks = folders.map(function(folder) {

      return gulp.src(path.join(scriptsPath, folder, '/**/*.js'))

        // concat into foldername.js

        .pipe(concat(folder + '.js'))

        // write to output

        .pipe(gulp.dest(scriptsPath)) 

        // minify

        .pipe(uglify())    

        // rename to folder.min.js

        .pipe(rename(folder + '.min.js')) 

        // write to output again

        .pipe(gulp.dest(scriptsPath));    

   });



   // process all remaining files in scriptsPath root into main.js and main.min.js files

   var root = gulp.src(path.join(scriptsPath, '/*.js'))

        .pipe(concat('main.js'))

        .pipe(gulp.dest(scriptsPath))

        .pipe(uglify())

        .pipe(rename('main.min.js'))

        .pipe(gulp.dest(scriptsPath));



   return merge(tasks, root);

});