Basic use of Promise

let promise = new Promise((resolve,reject) = >{
    setTimeout(() = >{
        resolve(1)},1000)})//1. Call "success/failure" via thene. catch
promise.then(res= >{console.log('Successful callback',res)})
.catch(err= >{console.log('Failed callback',err)})

//2. Use the second argument then to handle "failed, callback"
promise.then(res= >{console.log('Successful callback',res)},
err= >{console.log('Failed callback',err)})

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Promise can handle callbacks through the method of THEN, which has two arguments, the first triggering a callback to resolve (success) and the second to reject (failure). It is recommended to use the catch notation to catch exceptions

Principle of Promise

1. PromiseIt’s actually a constructor that, when we call it, passesnew PromiseTo create an instance and process it through a chain call
2. new PromiseIs a synchronization task,PromisethethenThe method is aMicro tasksWhen the context is executed, the promise is executed immediately, while the then method is executed after the main thread is executed. (Here it isEvent loop)

Write a basic Promise by hand

    class WPromise {
      static PENDING = "pengding";
      static FUFILLED = "fulfilled";
      static REJECTED = "rejected";
      constructor(executor) {
        this.status = WPromise.PENDING;
        this.value = null;
        executor(this._resolve.bind(this), this._reject.bind(this));
      }

      _resolve(value) {
        if (this.status === WPromise.PENDING) {
          this.status = WPromise.FUFILLED;
          this.value = value; }}_reject(reason) {
        if (this.status === WPromise.PENDING) {
          this.status = WPromise.REJECTED;
          this.value = reason; }}then(res, req) {
        if (this.status === WPromise.FUFILLED && res) {
          res(this.value);
        }
        if (this.status === WPromise.REJECTED && req) {
          req(this.value); }}}new WPromise((resolve, reject) = > {
      resolve(5);
      resolve(6);
      reject(7);
    }).then(
      (res) = > {
        console.log("Success", res);
      },
      (err) = > {
        console.log("Failure", err); });Copy the code