Question

为了在最近潜入节点后进行实验，我编写了一个简单的promise代码，然后从命令行执行它。我看到了两种输出。这是我的代码：

function doWork(){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            console.log('done!');
            resolve();
        }, 1000);
    });
}

然后：

   1. doWork().then(function(){
        return doWork();
    }).then(function(){
        console.log('that\'s it');
    });

Output :

 done!
 done!
 that's it!

另一种方式：

2. doWork().then(function(){
            doWork();
        }).then(function(){
            console.log('that\'s it');
        });

Output: 

done!
that's it!
done!

为什么在我没有return或我return时输出会发生变化？

Answer 1

承诺按返回值工作。当你没有Layer时，链不知道在继续链中的下一个处理程序之前等待第二个internalMethod()。

Answer 2

基本上，你通过不从成功处理程序返回来破坏承诺链。

doWork().then(function () {
    // this will be executed only after first call to doWork is resolved.
    // When this executes it returns the promise returned by doWork thereby asking next then handler to wait till this promise is resolved.
    return doWork();
}).then(function () {
    // since we are returning doWork second call promise, this will be executed only after it is resolved.
    // if we don't return doWork second call promise, it won't wait for promise to be resolved and directly execute.
    console.log('that\'s it');
});

执行时的承诺显示不同的结果

2 个答案: