code: function concatJS() {
      let tasks = [];
  
      Object.keys(config).forEach((key) => {
          let val = config[key];
          tasks.push(gulp.src(val.src)
              .pipe(plumber({ errorHandler: handleErrors }))
              .pipe(concat(key + '.js'))
              .pipe(gulp.dest(val.dest)));
      });
  
      return merge(tasks);
  }
docs: Concatenate docs JS files.