Handwritten simple promise

export class newPromise {
    
    constructor(executor) {
    //new  example   Pass in a function 
        executor(this.resolve, this.reject)
    }
    status = PENDING
    res = undefined
    result = undefined
    resolve = (value) => {
    //resolve Method   take pendding  become fulfilled
        if(this.status !== PENDING) return
        this.status = FULFILLED
        this.res = value
    }
    reject = result => {
    //reject  Method ： take pendding  become reject
        if(this.status !==PENDING) return
        this.status = REJECTED
        this.result = result
    }
    then = (successCallback,failCallBack) => {
    //then Method 
        if(this.status === FULFILLED) {
    
            successCallback(this.value)
        }else if(this.status == REJECTED) {
    
            failCallBack(this.result)
        }
    }
}

Promise Execute as soon as new .then Callback function specified by method , It will not be executed until all synchronization tasks of the current script have been executed ,

 In the above code ,promise Throw an error , I was catch() Method to capture . Be careful , The above is equivalent to the following two .

//  Writing a 
const promise = new Promise(function(resolve, reject) {
    
  try {
    
    throw new Error('test');
  } catch(e) {
    
    reject(e);
  }
});
promise.catch(function(error) {
    
  console.log(error);
});

//  Write two 
const promise = new Promise(function(resolve, reject) {
    
  reject(new Error('test'));
});
promise.catch(function(error) {
    
  console.log(error);
});

const someAsyncThing = function() {
    
  return new Promise(function(resolve, reject) {
    
    //  The next line will report an error , because x No statement 
    resolve(x + 2);
  });
};

someAsyncThing().then(function() {
    
  console.log('everything is great');
});

setTimeout(() => {
     console.log(123) }, 2000);
// Uncaught (in promise) ReferenceError: x is not defined
// 123
 In the above code ,someAsyncThing() The function produces  Promise  object , There's a syntax error inside . The browser runs to this line , It will print out an error message ReferenceError: x is not defined, But it doesn't exit the process 、 Terminate script execution ,2  It's still going to output in seconds 123. That is to say ,Promise  Internal errors do not affect  Promise  External code , The popular saying is “Promise  Will eat the mistake ”.

p = Promise.all Method ：  Will be multiple promise example   Pack into a new one promise  example 
 Take an array as an argument 
 Two cases ： 
	 Only when all the states become fulfilled p  Will become fulfilled  Returns an array 
	 Just one person  rejected  p  It becomes   reject  The first failed is returned 

p = Promise.rece() jiang duoge pormise  example   Pack into a new one pormise example 
	 As long as one instance changes state  p  The state of follows the high side , The return value of the first changed instance 
	 Parameter method and promise.all() yiyang  rug bushi pormise  example   Will call first  Promise.reoslve（）  Method   Convert parameter to  Promise  example   In further processing