promise的理解

//#1 返回promise实例给then

doSomething().then(()=>{

    return doSomethingElse();

})

//执行步骤

dosomething=>doSomethingElse(undefined)=>finalHandler(result of dosomethingElse)

//#2 不返回promise实例给then相当于返回空

doSomething().then(()=>{

    doSomethingElse();

})

//执行步骤

dosomething=>dosomethingElse(undefined)=>finalHandler(undefined)

//#3then里面立即执行函数返回的是promise实例，且和dosomething 同时执行,在同一个栈中执行

doSomething().then(doSomethingElse());

//执行步骤

dosomething()            =>    finalHandler(result of dosomething)

dosomethingElse(undefined)=>

//#4 dosomethingElse作函数传进去,then中默认可以传两个函数作为参数（resolve，reject)

doSomething().then(doSomethingElse);

//执行步骤

dosomething=>dosomethingElse(result of dosomething)=>finalHandler(result of dosomethingElse)