Async /await

The async function is the syntactic sugar of the Generator and the await command is the syntactic sugar of the internal THEN command.

Async is represented with the keyword async and await is used inside the function.

I. Improvements over Generator

1. Built-in actuator

The execution of Generator functions must depend on the executor, whereas Aysnc functions have their own executor and are called in the same way as normal functions

2, better applicability

By convention of the CO module, a yield command can only be followed by a Thunk function or a Promise object. The await command of async functions can be followed by a Promise or a primitive value (Number, string, Boolean, but this is equivalent to a synchronous operation).

The return value is Promise

Async functions return Promise objects, which are more convenient than iterators returned by Generator functions and can be called directly using the then() method

2. Use of Async

This is the part where I can’t think of anything to write and for those of you who know Promise and Generator, this is the part that can be solved in five seconds, right

Async error handling

Await can directly obtain the parameters passed by the subsequent Promise success state, but cannot capture the failure state

But, Promise can run as reject, so it’s better to put await in try… In the catch block

async function foo() {
	try {
		return await dosomething()
	} catch (err) {
		console.log(err)
	}
}
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When an async function has an await state, the following await is not executed.

Iv. Notes for Async

Await can only be used within async functions, with promises only, not for callbacks

2. Async operations after Await should be triggered at the same time if they do not depend on each other

async function foo() {
  let a = await request(url1);
  let b = await request(url2);
  let c = await request(url3);

  return a + b + c
}
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There is no connection between a, B, and c, which should be executed concurrently with promise.all ()

async function foo() {
  let results = await Promise.all([request(url1),request(url2),request(url3)])
  return results.reduce((total,item) => total * item)
}
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How to use async