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Discussion on: Modernize your ReactJS application with async/await in 2018

domysee profile image
Dominik Weber

Did you have a look at the transpiled JS code?

While async/await is a great feature, the size of the transpiled code is enormous.

The simple onLogin method of your example gets transpiled to this:

var onLogin = function () {
    var _ref = _asyncToGenerator( /*#__PURE__*/regeneratorRuntime.mark(function _callee() {
        var _state, email, password, response;

        return regeneratorRuntime.wrap(function _callee$(_context) {
            while (1) {
                switch (_context.prev = {
                    case 0:
                        _state = undefined.state, email =, password = _state.password;
                        _context.prev = 1;
               = 4;
                        return'/login', { email: email, password: password });

                    case 4:
                        response = _context.sent;

               = 10;

                    case 8:
                        _context.prev = 8;
                        _context.t0 = _context['catch'](1);

                    case 10:
                    case 'end':
                        return _context.stop();
        }, _callee, undefined, [[1, 8]]);

    return function onLogin() {
        return _ref.apply(this, arguments);

So before using async/await, one should ask if the increased code size is acceptable.

codeprototype profile image
Kevin Le Author

It's good to have an eye on the transpiled code. For a simple case, it might seem like there's no gain of using async/await over using then clauses. But the benefits of using async/await will be more obvious when the business logic gets more complicated. Async/await code will be more readable than "then" clauses.

Now if you have to stop each time to compare the size of the transpiled code, then I recommend against that. I also recommend against coming up with a rule that says something like "Ok, less than 2 "then" clauses, then no async/await".
Business logic does change. So suddenly we have to make one more asynchronous call, then such rule is more trouble than it's worth.

nickytonline profile image
Nick Taylor (he/him)

It depends what browsers you're targeting. Assuming (and I hope) you use babel-preset-env, async/await will only be transpiled to ES5 for pretty much just IE11 and below. All the other browsers support await now.

If most of your market is IE <= 11 , then maybe perhaps it's not wise to use async/await. Honestly though so many things come in to play for bundle size, but obviously a smaller bundle size is usually a good thing.

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domysee profile image
Dominik Weber

This is great news! Since it's ES7 I assumed it won't be implemented by browsers yet.

Thanks for pointing out that it is.

Regarding the bundle size, I agree that many things affect that. But I don't think that bloating up async/await statements to 10x the original code size is negligible.