folder concat

gulpfile.js

// pseudocode but something like this
var fs = require('fs');
var path = require('path');
var gulp = require('gulp');
var concat = require('gulp-concat');
var gif = require('gulp-if');

function getFolders(dir){
    return fs.readdirSync(dir)
      .filter(function(file){
        return fs.statSync(name).isDirectory();
      });
}

gulp.task('js', function(){
  var folders = getFolders("./parent-folder");
  var src = gulp.src("./parent-folder/**/*.js");
  var dest = gulp.dest("build/whatever");
  folders.forEach(function(folder){
    src
      .pipe(gif(folder+"/*.js", concat(folder+".js")))
      .pipe(dest);
  });
  return dest;
});