var ifElse = require('gulp-if-else');

// your code ...

gulp.src(source)
  .pipe( ifElse(condition, ifCallback, elseCallback) )

if(condition) {

  // condition is truthy

  return ifCallback();
}else{

  // condition is falsy

  // if "elseCallback" is provided
  if(elseCallback) {
    return elseCallback();
  }

  // if not "elseCallback" returns the stream
  return stream;
}

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

  gulp.src('./public/css/*.css')
    .pipe(ifElse(process.env.NODE_ENV === 'production',

      // called if "NODE_ENV" is "production"
      minifyCSS,

      // called if "NODE_ENV" is "not" "production" (else)
      function() {
        return minifyCSS({debug: true});
    }))
    .pipe(gulp.dest('./dist/css'))
});

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

  var isDev = process.env.NODE_ENV === 'development';

  gulp.src('./public/js/app.js')
    .pipe(browserify())

    // here, "uglify" (function) is called only if "isDev" is "true"
    .pipe(ifElse(isDev, uglify))
    .pipe(gulp.dest('./dist/js'))
});