Detailed explanation of es6-promise object

1.promise简介

promise是es6A new object in ,It is mainly used to obtain the result of the success or failure of an asynchronous operation.promise是jsCore objects in asynchronous programming,es8中await和async也是基于promise实现的

promise对象有3种状态,

状态变更：只有2种
pending => resolved
pending => rejected

2.promise的优点

避免回调地狱,promise对象执行完后,可以在then方法,或catch方法中,获取结果,thenmethod will also return onepromise对象（Even if it returns a normal object itself, it is eventually wrappedpromise对象）

3.promise对象的创建

let promise = new Promise((resovle,reject)=>{
    
    console.info("create promise object");
    setTimeout(() => {
    
        resovle("ok");
    });
}).then(value=>{
    
    console.info("successed to promise,returned value:"+value);
},reason=>{
    
    console.info("failed to execute promise,reason"+reason);
}).catch(reason=>{
    
    console.error(`failed to reason:${
      reason}`);
});;

promise.then(value=>{
    
    console.info("then:"+value);
});
console.info("end js ......");

promiseThe object's argument is a single arrow function,箭头函数2The input parameter is2个函数,resolve,reject,Status change for success or failure.
resolve的结果为thenThe first arrow function of the methodvalue参数,
如果then设置了第二个参数,则promise在被rejector if an exception is thrownreason的箭头函数.

then也会返回一个新的promise对象.
thenThe method will wait until the synchronized block of code is executed,才会执行.而promiseThe instantiated code is executed synchronously.
The entire execution is asynchronous.

The result of executing the above code is as follows：

4.promise常用静态方法

1.promise的对象方法(p1,p2,p3为promise的实例对象)
Promise.all()并发处理多个异步任务,所有任务都执行完成才能得到结果

Promise.all( [p1,p2,p3] ) .then ( (result) => {
    console.info (result);
})

2.Promise.race()并发处理多个异步任务,The result is returned as long as one task completes
Promise.all()并发处理多个异步任务,所有任务都执行完成才能得到结果

Promise.race ( [p1,p2,p3] ).then ( (result)=>{
    
console.log (result)
})