Promise小结

Primise 总结 js

回调

1. 同步回调

   func add(a,b,func){
     func(a+b)
   };
   console.log('start');
   add(1,2,v=>console.log(v));
   console.log('end');
   

2. 异步回调

   func add(a,b,func){
      setTimeout(()=>func(a+b),1000);
   };
   console.log('start');
   add(1,2,v=>console.log(v));
   console.log('end');3.
   

3. 回调

```javascript
// 嵌套回调
func1(function(v1)) {
    func2(function(v2)){
        func3(function(v3)){
            console.log(v1+v2+v3);
        }
    }
}
// Promise 回调
func1().then(function(){
    return v1;
}).then(function(obj){
    return v1+obj;
}).then(function(obj){
    return v3 + obj;
})
```

Promise 使用

• resolve & reject

  promise构造函数生成Promise对象，入参函数resolve，reject

  var promise = new Promise(function(resolve,reject){
    // if succ
    resolve('succ');
    // if err
    reject('err');
  })
  

• Fulfilled & Rejected

  Promise对象一开始是Pending的准备状态

  执行了resolve()，状态变为onFulfilled状态

  执行了reject()，状态变为onRejected状态

• 便捷构造方法

  Promise.resolve(1);
  // ==
  new Promise((resolve) => resolve(1));
  

  Promise.reject(new Error("BOOM")) 
  // ===
  new Promise((resolve, reject) => reject(new Error("BOOM")));
  

• 异常捕获 catch

  catch方法可以捕获到then中抛出的错误

  var promise = Promise.resolve();
  promise.then(function() {
          return 'success';
  }).then(function (success) {
      console.log(success);
      throw new Error("Another BOOM!");
  }).catch(function (error) {
      console.log(error); // 可以正常捕捉到异常
  });