Implement A simple Promise that conforms to the Promise/A+ specification

In a nutshell: Promises allow us to solve the “callback hell” problem by chain-calling, especially in asynchronous processes, and Promise to keep code clean and readable.

First, the use of promises

        var foo = new Promise(function (resolve, reject) {
            setTimeout(function () {
                resolve("success")}, 1000); console.log("Make a Promise"); }) foo.then((res) => { console.log(res) }).catch((err) => { console.log(err); }) // Output: create a Promise successCopy the code

Here is an example of a promise, and the output reads, “Create a Promise”, and after a 1000ms delay, “success” is printed.

As you can see from the examples above, promises are convenient for handling asynchronous operations. In addition, promises can be invoked chained.

Ii. Promise/A+ specification

(1) A promise must have three states: Pending, disappointingly (resolved) and Rejected.

When you are in the pending state, you can switch to the regrettable or Rejected state. When you are in a regrettable state or the rejected state, there is no change.

(1) A promise must have a THEN method, which accepts two parameters: PROMISE. Then (onFulfilled,onRejected)

The onFulfilled method represents the method to be executed when the state changes from Pending — >fulfilled(resolved)

OnRejected indicates the method whose state is executed from pending — > Rejected.

Iii. Fulfill the Promise in accordance with the Promise/A+ specification

function myPromise(constructor){
    let that=this;
    that.status="pending"That. Value =undefined; That. Reason =undefined; // Define the state of the rejected statefunction resolve(value){
       if(that.status==="pending"){
          that.value=value;
          that.status="resolved"; }}functionReject (reason){// Two === ="pending", ensuring that the change in state is irreversibleif(that.status==="pending"){
          that.reason=reason;
          that.status="rejected"; Constructor (resolve,reject); }catch(e){ reject(e); }}Copy the code

myPromise.prototype.then=function(onFullfilled,onRejected){
   let that=this;
   switch(that.status){
      case "resolved":
        onFullfilled(that.value);
        break;
      case "rejected":
        onRejected(that.reason);
        break;
      default:       
   }
}

Copy the code