[JS advanced] promise explanation

Back to hell

promise promise

Fulfill your promise Promises sink into the sea Promise to wait for the result ……

resolve solve the problem

reject Refuse to consider

pending Wait for

A means to solve asynchronous process .

Promise It's a constructor need new

Promise Parameters excutor actuator

excutor -> resolve reject function

excutor new Promise call

// excutor  It's synchronous 
let promise = new Promise((resolve,reject)=>{
    console.log(1);
})
console.log(2);

// then  Is an asynchronous call 

excutor It's synchronous .

then Is an asynchronous call .

pending -> resolve

pending -> reject

Only in this way .

Promise.all([promise1,promise2,promise3])

Multiple asynchronous tasks run concurrently , Its results are used after creating commitments , Wait for all task results to be completed .

iterable The internal elements pass promise A collection of objects , If not promise, direct resolve Return the value to

iterable There are no elements inside , Just return an empty array .

Just one of them promise yes rejected In terms of state , Then the instance callback is rejected.

The reason for the failure is the first one promise Result

Promise.race()

Whoever finishes first will return to that promise Result , Success or failure (fullfilled perhaps rejected)

Test the response speed of resources or interfaces

async await

await Is an operator , Waiting for one Promise The operation means for the object to produce results

Function is pause async Execution of a function , wait for promise The result of the treatment .

If Promise The result is rejected, It throws an exception

acync Functions are implicit Promise return pending state

async It means that the current asynchronous function and the program under the same scope are asynchronous .