A nice async / await, generator / yield

tags: web

async/await
async function is a function of the async keyword declared. async AsyncFunction function is an example of the constructor, and which allows await keyword. async and await keywords so that we can use a more concise way to write asynchronous behavior based Promise, without deliberately chained calls promise.
await operator to wait a Promise object. It can only be used in asynchronous function async function.

Code Example:

function resolveAfterManySeconds(duration) {
  return (new Promise(resolve => {
    setTimeout(() => {
      resolve(duration);
    }, 1000*duration);
  })).then(function(x){
  	return x*100;
  });;
}

async function f1() {
	console.log(new Date());

	var x = await resolveAfterManySeconds(1);
	var y = await resolveAfterManySeconds(4);
	console.log(x); // 1
	console.log(y); // 4

	console.log(new Date());
}

f1(); // Reference console.log, 5 seconds Back

The results demonstrate:

Sun Feb 21 2021 16:49:50 GMT+0800 (China Standard Time)
async.html:24 100
async.html:25 400
async.html:27 Sun Feb 21 2021 16:49:55 GMT+0800 (China Standard Time)

generator/yield
This function * declaratively (function keyword followed by an asterisk) defines a generator function (generator function), which returns a Generator object.
yield keyword to pause and resume a generator function ((function * legacy or generator function).

Code Example:

function* countAppleSales () {
  var saleList = [3, 7, 5];
  for (var i = 0; i < saleList.length; i++) {
    yield saleList[i];
  }
}

var appleStore = countAppleSales(); // Generator { }
console.log(appleStore.next()); // { value: 3, done: false }
console.log(appleStore.next()); // { value: 7, done: false }
console.log(appleStore.next()); // { value: 5, done: false }
console.log(appleStore.next()); // { value: undefined, done: true }

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