JavaScript/Node promise : Executing them in sequence

Question

我有点迷茫，我想我遇到了“只见树木不见森林综合症”。

我是一个 JS 新手，我正在尝试了解如何按顺序调用一组 JS 函数(返回 promises)。我做了一些阅读，并决定鉴于我使用的是 Node，我应该使用像 bluebird 这样的东西来管理 promise 。

我无法解决的是为什么这段代码不起作用

var Promise = require("bluebird");
// My Promise enabled function from oReily Safari book
function countdown(seconds, timername) {
    return new Promise(function (resolve, reject) {
        console.log('Countdown : Starting Countdown ' + timername);
        for (let i = seconds; i >= 0; i--) {
            setTimeout(function () {
                if (i > 0)
                    console.log(timername + ' ' + i + '...');
                else
                {
                    console.log('countdown '+timername+' now=='+i+' resolving!');
                    resolve(console.log("Countdown : timename="+timername+" ended"));
                }
            }, (seconds - i) * 1000);
        }
    });
}

/*Basic test of promise */
/* when this is run I expected countdown(5, 'Basic : Timer1') to execute and then when **resolved** THEN countdown(5, "Basic Timer2") to execute. 
 *however what I see is both timers executing at the same time.. 
 */

console.log('Basic : Countdown promise test');
countdown(5, 'Basic : Timer1').
        then(function ()
        {
            /*Success */
            console.log("Basic : Ended Successfully");
        }).
        then(countdown(5, "Basic : Timer2")
                );

当我运行它时，我希望 countdown(5,'timer1') 先执行，然后，只有当 timer1 完成时，timer2 才会执行。

但是当我运行它时我得到了

Basic : Countdown promise test
Countdown : Starting Countdown Basic : Timer1
Countdown : Starting Countdown Basic : Timer2
Basic : Timer1 5...
Basic : Timer2 5...
Basic : Timer1 4...
Basic : Timer2 4...
Basic : Timer1 3...
Basic : Timer2 3...
Basic : Timer1 2...
Basic : Timer2 2...
Basic : Timer1 1...
Basic : Timer2 1...
countdown Basic : Timer1 now==0 resolving!
Countdown : timename=Basic : Timer1 ended
countdown Basic : Timer2 now==0 resolving!
Countdown : timename=Basic : Timer2 ended
Basic : Ended Successfully
Done.

我迷路了..

提前致谢

Answer 1

你代码的最后一部分有一个意想不到的错误:

    then(countdown(5, "Basic : Timer2") );

这意味着 countdown() 的结果用作回调函数。 (直接执行倒计时功能)

改为使用

then(function(lastResult){ countdown(5, "Basic : Timer2") });

变量 lastResult 将具有链中较早 promise 的返回值。