Using promises

【Using promises】

　　过去，异步方法这样写：

function successCallback(result) {
  console.log("It succeeded with " + result);
}

function failureCallback(error) {
  console.log("It failed with " + error);
}

doSomething(successCallback, failureCallback);

　　使用promise后，异步方法这样写：

let promise = doSomething(); 
promise.then(successCallback, failureCallback);

　　promise模式有以下几个好处：

　　1、回调方法永远在下一帧后才会调用。即使当前帧已完成。

　　2、可以通过串连.then,设置多个回调。

　　promise模式可以用于解决死亡金字塔问题：

doSomething(function(result) {
  doSomethingElse(result, function(newResult) {
    doThirdThing(newResult, function(finalResult) {
      console.log('Got the final result: ' + finalResult);
    }, failureCallback);
  }, failureCallback);
}, failureCallback);

doSomething().then(function(result) {
  return doSomethingElse(result);
})
.then(function(newResult) {
  return doThirdThing(newResult);
})
.then(function(finalResult) {
  console.log('Got the final result: ' + finalResult);
})
.catch(failureCallback);

　　catch(failureCallback) 是 then(null, failureCallback) 的简单写法

　　使用new Promise((resolve, reject)=>{})创建 Promise。Basically, the promise constructor takes an executor function that lets us resolve or reject a promise manually

const wait = ms => new Promise(resolve => setTimeout(resolve, ms));

wait(10000).then(() => saySomething("10 seconds")).catch(failureCallback);

new Promise((resolve, reject) => {
    console.log('Initial');

    resolve();
})
.then(() => {
    throw new Error('Something failed');
        
    console.log('Do this');
})
.catch(() => {
    console.log('Do that');
})
.then(() => {
    console.log('Do this whatever happened before');
});

　　Promise.resolve() and Promise.reject() are shortcuts to manually create an already resolved or rejected promise respectively

　　以下两段代码均为reduce应用。用于串于执行异步或同步调用。

[func1, func2].reduce((p, f) => p.then(f), Promise.resolve());

Promise.resolve().then(func1).then(func2);

参考：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises