使用

  1. var p = new Promise(function(resolve, reject){
  2.     console.log('执行')
  3.     setTimeout(function(){
  4.         resolve(2)
  5.     }, 1000)
  6. })
  7. p.then(function(res){
  8.     console.log('suc',res)
  9. },function(err){
  10.     console.log('err',err)
  11. })

手写 Promise

  1. function Promise(executor) {
  2.   var _this = this;
  3.   this.status = 'pending'; // 状态
  4.   this.value = undefined; // 成功结果
  5.   this.reason = undefined; // 失败原因
  7.     function resolve(value){ this.state = 'fullfilled' }
  8.     function reject(reason){ this.state = 'rejected' }
  10.   executor(resolve, reject);
  11. }
  13. Promise.prototype.then = function(onFullfilled, onRejected){
  14.     if(this.state === 'fullfilled'){
  15.       onFullfilled(this.value)
  16.   }
  17.   if(this.state === 'rejected'){
  18.     onRejected(this.reason)
  19.   }
  20. }
  22. module.exports = Promise;

promise.all

  1. Promise.all = (promises) => {
  2.     return new Promise((rs, rj) => {
  3.       let count = 0;
  4.     let len = promises.length;
  5.     let result = [];
  7.     if(len === 0) return rs([])
  9.     promises.forEach((item, index) => {
  10.         Promise.resolve(p).then(res => {
  11.           count +=1;
  12.         result[i] = res;
  13.         if(count === len) return rs([])
  14.       })
  15.     }).catch(rj)
  16.   })
  17. }

取消Promise方法

  1. 当新对象保持“pending”状态时，原Promise链将会中止执行。 ```javascript Promise.resolve().then(() => { console.log(‘ok1’) return new Promise(()=>{}) // 返回“pending”状态的Promise对象 }).then(() => { // 后续的函数不会被调用 console.log(‘ok2’) }).catch(err => { console.log(‘err->’, err) })
  2. 2. Promise.race竞速方法
  3. ```javascript
  4. let p1 = new Promise((resolve, reject) => {
  5.     resolve('ok1')
  6. })
  8. let p2 = new Promise((resolve, reject) => {
  9.     setTimeout(() => {resolve('ok2')}, 10)
  10. })
  12. Promise.race([p2, p1]).then((result) => {
  13.     console.log(result) //ok1
  14. }).catch((error) => {
  15.     console.log(error)
  16. })

**3. 当Promise链中抛出一个错误时，错误信息沿着链路向后传递，直至被捕获

  1. Promise.resolve().then(() => {
  2.     console.log('ok1')
  3.     throw 'throw error1'
  4. }).then(() => {
  5.     console.log('ok2')
  6. }, err => {     
  7.     // 捕获错误
  8.     console.log('err->', err)
  9. }).then(() => {   
  10.     // 该函数将被调用
  11.     console.log('ok3')
  12.     throw 'throw error3'
  13. }).then(() => {
  14.     // 错误捕获前的函数不会被调用
  15.     console.log('ok4')
  16. }).catch(err => {
  17.     console.log('err->', err)
  18. })