promise function

1. The original intention of promise:

          1. Realize asynchronous operation

          2. Solve callback hell

           Promise is a constructor , which has reject, resolve, and race methods, and then and catch methods on the prototype chain. Using new to create a promise object will have these methods.

            1. It is designed as a global constructor

            2. The object created by this function is a special data container

            3. There are three states inside the data container: Waiting ==> correct data generated Wrong data generated

            4. When creating an object, it is required to pass a function parameter ==> that is, to process time-consuming business in this function. General usage:

new Promise((resolve,reject)=>{resolve(),reject()}) 

2. The then and catch methods in the promise function

       Initial state: pending ---> successful state: fulfilled call resolve

       Initial state: pending ---> failure state: rejected call reject

promise object.then(function(res){}): Execution succeeded

              The res in the callback function in the first then method represents the parameter value of the success or failure (resolve, reject) function

              promise object.catch(function(err){}): Execution failed

              The res parameter in the then method is the return value of the previous then method

  let pro1 = new Promise(function(resolve, reject) {
            // resolve  成功
            // reject   失败
            if (1) {
                // resolve("成功啦！");
                resolve(2);
            } else {
                reject("失败啦！");
            }
        })

    pro1.then(res => {
            console.log(res);
            return "我是第一个then方法的返回值"
        })
        .then(res => {
            console.log(res);
            return "我是第二个then方法的返回值"
        })
        .then(res => {
            console.log(res);
        })
        .catch(err => {
            console.log(err);
        })
    console.log(3);