Using gulp with google closure to compile js files into individual files instead of a single file

As far as I can see there's no way to avoid having your files combined by the Google Closure Compiler. Even if you leave out the js_output_file option it will simply default to the output file name compiled.js. That means you have to invoke the Closure Compiler for each file.

The gulp-flatmap plugin lets you do that easily. You just have to determine the value of js_output_file for each input file (basically replace .js with .min.js in each file name):

var flatmap = require('gulp-flatmap');
var path = require('path');
gulp.task('js-closure', function () {
  return gulp.src('app/js/*.js', {base: './'})
    .pipe(flatmap(function(stream, file) {
      return stream.pipe(closureCompiler({
        compilation_level: 'SIMPLE_OPTIMIZATIONS',
        warning_level: 'QUIET',
        language_in: 'ECMASCRIPT6_STRICT',
        language_out: 'ECMASCRIPT5_STRICT',
        output_wrapper: '(function(){\n%output%\n}).call(this)',
        js_output_file: path.basename(file.path).replace(/js$/, 'min.js')
      }))
    }))
    .pipe(gulp.dest('./dist/js'));
});