vjeux / asyncawait

Async Await

Using the AsyncAwait pattern, you can write any asynchronous function with a callback as last argument in a synchronous way.

Example

var multiply = function (x, y, callback) {
  callback(x * y);
}

Now, we want to do 1 * 2 * 3 * 4 using an asynchronous multiply function:

function main() {
  multiply(1, 2, function (res) {
    multiply(res, 3, function (res) {
      multiply(res, 4, function (res) {
        console.log(res);
      })
    })
  })
}
main() // 24

We just wrote spaghetti code. Now let's look the same code with async, await and yield:

var main = async(function () {
  var [res] = yield await(multiply)(1, 2);
  [res] = yield await(multiply)(res, 3);
  [res] = yield await(multiply)(res, 4);
  console.log(res);
});
main() // 24

Using async and await, we can easily combine asynchronous functions to write a new one:

var factorial = async(function (n, callback) {
  var res = 1;
  for (; n > 0; --n) {
    [res] = yield await(multiply)(res, n);
  }
  callback(res);
});

We can either use it the old way:

var main = function () {
  factorial(4, function (res) {
    console.log(res);
  })
}
main() // 24

or the new way:

var main = async(function () {
  var [res] = yield await(factorial)(4);
  console.log(res);
});
main() // 24

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