setTimeout在forEach中不起作用 [英] setTimeout not working inside forEach

问题描述

我有一个forEach调用函数.每次调用之间必须有一个延迟.我已经将它放在forEach中的setTimeout中.第一次等待后不考虑超时.而是等待一次，然后一次运行.我已将超时设置为5秒，并且正在使用控制台进行确认.等待5秒，然后一次将所有foobar控制台日志全部记录.

I have a forEach that calls a function. There needs to be a delay between each time it is called. I've put it inside a setTimeout inside the forEach. It isn't respecting the timeout after the first wait. Instead it is waiting once, then running all at once. I've set the timeout to 5 seconds and I am using a console to confirm. 5 seconds of wait, then several foobar console logs all at once.

为什么会出现这种情况?

var index = 0;
json.objects.forEach(function(obj) {
    setTimeout(function(){
        console.log('foobar');
        self.insertDesignJsonObject(obj, index);
    }, 5000);
});

推荐答案

Jason所说的完全正确，但我想我会试一试，以便更好地阐明.

What Jason said is totally correct in his answer but I thought I would give it a shot, to better clarify.

这实际上是一个经典的关闭问题.通常情况如下:

This is actually a classic closure problem. Typically it would look something like:

for(var i = 0; i < 10; i++){
    setTimeout(function(){
        console.log(i);
    },i * 1000)
}

新手希望控制台显示:

0
(0 seconds pass...)
1
(1 second passes...)
2
(etc..)

但是事实并非如此！您实际看到的是数字10被记录10次(每秒1x)！

But that just isn't the case! What you would actually see is the number 10 getting logged 10 times (1x per second)!

为什么会这样?"好问题.封闭范围.上面的for循环缺少闭包范围，因为在javascript中，只有函数(lambda)具有闭包范围！

"Why does that happen?" Great question. Closure scope. The for loop above lacks closure scope because in javascript, only functions (lambdas) have closure scope!

请参阅: https://developer.mozilla.org/zh-美国/docs/Web/JavaScript/关闭

但是！如果您尝试过以下操作，那么您的尝试将获得所需的输出:

However! Your attempt would have achieved the desired output if you had tried this:

    json.objects.forEach(function(obj,index,collection) {
        setTimeout(function(){
            console.log('foobar');
            self.insertDesignJsonObject(obj, index);
        }, index * 5000);
    });

因为您可以访问"closur-ed" index变量-可以在调用函数(lambda)时将其状态视为期望状态！

Because you have access to the "closur-ed" index variable - you can rely on its state being the expected state when the function (lambda) is invoked!

其他资源:

JavaScript闭包如何工作?

http://javascript.info/tutorial/closures

http://code.tutsplus.com/tutorials /closures-front-to-back--net-24869

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