The difference between asyncawait and promise

async

async function definition

Async functions are functions declared using the keyword.async is short for "asynchronous", so it should be well understood that async is used to declare that a function is asynchronous.

Function

Used to solve: bugs generated by asynchronous programs

####async function syntax

async function name([param[, param[, ... param]]]) {statements}

Parameter: name function name

param The name of the parameter to pass to the function

statements contains the expression of the function body, you can use await

Return value: A brand new promise that will either be resolved with a value returned by the async function, or rejected with an exception thrown (or uncaught) from the async function

The following is a specific use case of async:

 async function foo(p) {console.log("foo run",p);return 1;}var res = foo(1);console.log(res);//{: 1}

Async functions must return a promise object.If the return value of an async function does not appear to be a promise, then it will be implicitly wrapped in a promise.

For example, the following code:

async function foo() {//promise.[[promiseValue]]return 1}//Equivalent tofunction foo() {return Promise.resolve(1)}//{: 1}

Here are some additions to async:

• async itself is a syntactic sugar—>Syntax sugar: keywords with certain functions

  • Syntax sugar
  The role of a>: It can reduce the amount of code and increase the readability of the program, thereby reducing the chance of program code errors

How to accept return value in async function

Method One

Get through promise.then-cb parameter

 async function foo(){console.log(222222);return 123;}let res = foo();res.then(data=>{console.log(data);//123})

Method 2

The second way to accept the return value of a function is await

 (async function (){console.log('async run');//The second way to accept the return value of a function is awaitlet res = await foo();console.log(res);})()async function foo(){console.log('foo run');return 123;}//res role: accept the return value of async foo function is promiselet res = foo();

await

await definition

await means to wait, so it should be well understood that await waits for an operation to complete.

Function

The function of the await keyword is to get the content returned in the Promise, and get the value of resolve or reject in the Promise function (the function of await is to get the value of promise.[[promiseValue]])

Notes about await

• await must be written in async

• await is followed by a promise instance object

[[promiseValue]]

[[PromiseValue]] is an internal variable, which cannot be obtained externally, but can only be obtained in then.

[promiseValue]Which can be assigned

• return of async function
• Resolve arguments in new promise
• return in then (return in catch finally)
• promise.reslove() argument promise.reject() argument

The difference between the three

• Promises and async/await are both ways of solving asynchronous programming, but async/await makes asynchronous code look like synchronous code.
• There is an async keyword in front of the function.The await keyword can only be used inside async functions.The async function implicitly returns a Promise, and the resolve value of the promise is the return value.

Why async/await is better?

Using the async function can make the code much simpler, it doesn't need then like Promise, no need to write anonymous function to deal with the value of Promise's resolve, no need to define redundant data variables, and nested code is avoided.

async/await allows try/catch to handle both synchronous and asynchronous errors.In the example below, try/catch can't handle JSON.parse errors, because it's in Promise, we need to use .catch, such errors will make the code very redundant.

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