gulpfile.js

var browserify = require('browserify');
var watchify = require('watchify');
var gutil = require('gulp-util');
var fs = require('fs')

var files = {
    js: {
        src: 'path to src',
        dst: 'path to dst'
    }
};

gulp.task('js-watch', function () {
    var args = watchify.args;
    args.degub = true;
    var bundler = watchify(browserify(files.js.src, args));

    bundler.transform('babelify');
    bundler.on('update', rebundle);

    function onError(e) {
        gutil.log(gutil.colors.red(e.message));
    }

    function rebundle() {
        var start = Date.now();

        return bundler.bundle()
          .on('error', onError)
          .on('end', function () {
              var time = Date.now() - start;
              gutil.log('Building \'' + gutil.colors.green(files.js.src) + '\' in ' + gutil.colors.magenta(time + ' ms'));
          })
          .pipe(fs.createWriteStream(files.js.dest));
    }

    rebundle();

});

package.json

{
  "dependencies": {
    "babelify": "^6.1.2",
    "browserify": "^10.2.4",
    "gulp": "^3.9.0",
    "gulp-util": "^3.0.6",
    "react": "^0.13.3",
    "watchify": "^3.2.3",
  }
}