Javascript 循环 - 等待值

Question

在所有代码运行之前完成函数的噩梦。我正在尝试构建一个计数器，并且仅在代码完成时返回。

我已经这样模拟了(我知道这并不好，但如果有人能指出我正确的方向，我将不胜感激):

//I want this to alert "Done"
alert(timerCheck());

function timerCheck() {
    var finished;
    var myLoop = 5;
    for (i = 0; i < myLoop; i++) {
        //This is emulating the slow code
        window.setTimeout(checkFinished, 900);
        alert(i);
    }
    function checkFinished() {
        //I originally had the "return here, before realising my error
        finished = true;
    }
    if (finished) {
        //This is where my problem is 
        return "done";
    }
}

就像我说的，一个非常简化的例子 - 如果有人能指出错误，我会省去很多麻烦!

Answer 1

如果函数调用并依赖于异步函数，则无法同步获取函数的返回值。

您必须使用回调。参见 this question了解更多详情。

例如，您的函数如下所示:

// pass a callback which gets the result of function
timerCheck(function(result) {
    alert(result);
});

function timerCheck(callback) {
    var myLoop = 5,
        j = 0;

    for (var i = 0; i < myLoop; i++) {
        // This is emulating the slow code
        // this will invoke `checkFinished` immediately, 
        // after 900ms, after 1800ms, after 2700ms and after 3600ms
        window.setTimeout(checkFinished, i * 900);
    }

    function checkFinished() {
       j++;
       // after checkFinish was called 5 times, we invoke the callback
       if(j === 5) {
           callback('done');
       }
    }
}