1.Promise
Promise is a popular asynchronous solution, essentially a constructor.
1.1 state
Promise has three states, the initial state is PEDding, including:
- Pedding => Resolve
- If pedding fails, go to Reject
- Waiting status Pedding
Let’s start with a simple example
let promise = new Promise(function(resolve,reject){
//throw new Error('Wrong')
//resolve()
reject()
});
promise.then(()=>{
console.log('success')
},()=>{
console.log('error');
});
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Each promise instance has a THEN method, which contains two parameters: successful callback and failed callback, and multiple THEN, successful callback and failure of all successful methods. In essence, THEN is asynchronous, also known as microtask. Other methods include:
- Promise.all() All Promise objects trigger a success state if they succeed, or a failure state if they fail
- Promise.race() any sub-pormise state will be called immediately by the parent Promise after it is triggered, and one can be completed
2. Make a Promise yourself
All promises follow the promiseA+ specification
Simple version 2.1
function Promise(executor){
letself = this; self.value = undefined; // Define success value self.reason = undefined; // Define failed reson self.status ='pending'; // The initialization state is PEDdingfunction resolve(value){
if(self.status === 'pending'Self. value = value; self.value = value; self.status ='resolved'; }}function reject(reason){
if(self.status === 'pending'Self. reason = reason; self.status ='rejected'; }} try{executor(resolve,reject); }catch(e){reject(e); } } Promise.prototype.then =function(onFulfilled,onRejected){
letself = this; // Determine the current status // success statusif(self.status === 'resolved'// This is a big pity (self.value); // This is a big pity. // The example abovethenSuccess callback (console.log('success'} // In the failed stateif(self.status === 'rejected'){// If rejected (self.reason); // If rejected (self.reason); // The example abovethenFailure callback (console.log('error'Module. Exports = exports;Copy the code
2.2 Slightly improved
However, if an asynchronous method is added to a New Promise and the execution of resolve is delayed for 3 seconds, then will not execute the successful or failed method because the successful or failed methods need to be stored in an array
let promise = new Promise(function(resolve,reject){
setTimeout(()=>{
resolve()
},3000)
});
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To perfect the example
function Promise(executor){
let self = this;
self.value = undefined;
self.reason = undefined;
self.status = 'pending'; self.onResolvedCallbacks = []; / / storethenSelf.onrejectedcallbacks = []; / / storethenFailed callback infunction resolve(value){
if(self.status === 'pending'){
self.value = value;
self.status = 'resolved'; self.onResolvedCallbacks.forEach(fn=>fn()); // Loop executed successfully}}function reject(reason){
if(self.status === 'pending'){
self.reason = reason;
self.status = 'rejected'; self.onRejectedCallbacks.forEach(fn=>fn()); // loop execution failed}} try{executor(resolve,reject); }catch(e){ reject(e); // this is a big Promise, onFulfilled // this is a big Promise, onFulfilled // This is a big Promise, onFulfilled // This is a big Promise, onFulfilled // This is a big Promise, onFulfilled // This is a big Promisefunction(onFulfilled,onRejected){
let self = this;
if(self.status === 'resolved'){
onFulfilled(self.value);
}
if(self.status === 'rejected'){
onRejected(self.reason);
}
if(self.status === 'pending') {/ / when a wait state, save the callback function self. OnResolvedCallbacks. Push (() = > {onFulfilled (self. Value); }); self.onRejectedCallbacks.push(()=>{ onRejected(self.reason) }); } } module.exports = Promise;Copy the code
2.3 Chain call
We know that promises can go on and on to implement chain calls, but the example above clearly doesn’t work for example
let p = new Promise((resolve,reject) =>{
resolve(123)
})
let p1 = new Promise((resolve,reject) =>{
resolve(111)
})
p.then((data)=>{
console.log(data)
returnP1 // returns p1 as a new promise. Continue with success, passing 111 to the next newData. Failure is the samethen}). Then ((newData)=>{console.log()'p1',data)
})
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Improve the code
function Promise(executor) {
let self = this;
self.value = undefined;
self.reason = undefined;
self.status = 'pending';
self.onResolvedCallbacks = [];
self.onRejectedCallbacks = [];
function resolve(value) {
if (self.status === 'pending') {
self.value = value;
self.status = 'resolved'; self.onResolvedCallbacks.forEach(fn => fn()); }}function reject(reason) {
if (self.status === 'pending') {
self.reason = reason;
self.status = 'rejected';
self.onRejectedCallbacks.forEach(fn => fn());
}
}
try {
executor(resolve, reject);
} catch (e) {
reject(e);
}
}
/**
*
* @param {*} promise2 thenReturn value (the new promise returned) * @param {*} xthen* @param {*} resolve resolve of promise2 * @param {*} reject Reject */functionResolvePromise (promise2, x, resolve, reject) {/ / promise2 and function performs the returned result is the same object, after waiting for himself, can't <! - for instance,letp1 = p.then((data)=>{--> <! --returnP1 // Return a promise that will neither succeed nor fail, waiting for yourself --> <! -}) -- -- >if(promise2 === x){
return reject(new TypeError('Chaining cycle'));
}
letcalled; // x can be either a promise or a normal valueif(x! ==null && (typeof x==='object' || typeof x === 'function')){
try{
let then= x.then; // This promise may have been written by someone else, so try catch // x may still be a promiseif(typeof then= = ='function'){then.call(x,y=>{// returns a successful result after a promise // recurses until a normal value is resolvedif(called) return; // Prevent multiple calls to called =true; / / recursive may after the success of the result is a promise that is about to cycle to parse resolvePromise (promise2, y, resolve, reject); },err=>{// The result of a promise failureif(called) return;
called = true;
reject(err);
});
}else{
resolve(x);
}
}catch(e){
if(called) return;
called = true; reject(e); }}else{// If x is a constant resolve(x); } } Promise.prototype.then =function (onFulfilled, onRejected) {
let self = this;
letpromise2; // Each call is requiredthenPromise2 = new promise ((resolve, reject) =>thenA promise was returnedif (self.status === 'resolved') {
setTimeout(()=>{try {// An exception may occur when a successful callback is executed, so use this exception as the result of a promise2 errorletx = onFulfilled(self.value); // The new promise, i.ethenReturns the result / / return the result after execution of the current success callback may be promise resolvePromise (promise2, x, resolve, reject); } catch (e) { reject(e); }},0)} //setTimeout
if (self.status === 'rejected') {
setTimeout(()=>{
try {
letx = onRejected(self.reason); resolvePromise(promise2,x,resolve,reject); } catch (e) { reject(e); }}}, 0)if (self.status === 'pending') {/ / wait to also want to pack a promise2 self. OnResolvedCallbacks. Push (() = > {setTimeout(()=>{
try {
letx = onFulfilled(self.value); resolvePromise(promise2,x,resolve,reject); } catch (e) { reject(e); }}}, 0)); self.onRejectedCallbacks.push(() => {setTimeout(()=>{
try {
letx = onRejected(self.reason); resolvePromise(promise2,x,resolve,reject); } catch (e) { reject(e); }}}, 0)); }});return promise2
}
module.exports = Promise;
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