gulp-beautify doesn't work on source files

I'm kinda new to gulp, and I've been trying to beautify my js source files. I have this task (which runs 100%) which uses 'gulp-beautify' to beautify the js files:

gulp.task('js', function() {
    return gulp.src('src/js/**/*.js')
        .pipe(beautify({indent_size: 4}));
});

And it doesn't work. The file stays the same. But, if I do this:

gulp.task('js', function() {
    return gulp.src('src/js/**/*.js')
        .pipe(beautify({indent_size: 4}))
        .pipe(gulp.dest('./foo/'));
});

The files outputted are beautified. Any idea why this is happening?

Thanks in advance!

via NonameSL