用于将回调放置在nodej中的语法和方法

本文介绍了用于将回调放置在nodej中的语法和方法的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我使用快捷库在nodejs中有以下http端点：

I have the following http endpoint in nodejs using the express library:

app.get("/api/stocks/lookup/:qry", function(req, res) { getJson(lookupSearch(req.params.qry), function(json) { var quotes = []; und.forEach(json, function(d) { getJson(quoteSearch(d.Symbol), function(j) { quotes.push(j); }); }); res.send(quotes); //how can I make this execute after the .forEach is finished? }); });

这里， getJson 看起来像这样： / p>

Here, getJson looks like this:

var getJson = function(search, cb) { http.request(search, function(response) { var raw = ''; response.on('data', function(d) { raw += d; }); response.on('end', function() { cb(JSON.parse(raw)); }); response.on('error', function(err) { console.error(err); }); }).end(); };

我看到为什么这不会作为http请求 getJson 是异步的，因此 res.send（quotes）将立即发回。那么，如何在 forEach 循环完成后发送 res.send（quotes）我可以在 forEach 函数上附加回调吗？

I see why this won't work as the http requests inside getJson are asynchronous and so res.send(quotes) will be sent back almost immediately. So, how can I get res.send(quotes) to be sent after the forEach loop is complete. Can I attach a callback onto a the forEach function?

总之，

如何在 forEach 循环完成。

可以附加回调（例如在 forEach 循环后执行的回调）到对象？我可以附加回调到什么？要清楚，对我的回调的想法意味着事件循环将在其中附加回调的函数/对象完成执行之后调用它。

How can I use res.send(quotes) after the forEach loop is complete.

Is it possible to attach callbacks (such as a callback to be executed after the forEach loop) onto objects? What can I attach callbacks to? To be clear, the idea of a 'callback' to me means that the event loop will call it after the function/object in which the callback is attached to is finished executing.

感谢所有的帮助！

推荐答案

将getJson转换为promise将是一个好主意，因为承诺是很好的合作。没有promise，手动的方式是保持未完成请求的计数器：

Converting your getJson into a promise would be a good idea, as promises are nice to work with. Without the promises, the manual way is to keep a counter of outstanding requests:

var outstanding = 0; json.forEach(function(d) { outstanding++; getJson(quoteSearch(d.Symbol), function(j) { quotes.push(j); if (!--outstanding) { res.send(quotes); } }); });

如果你按promise的方式，你会做一个 code> over json ，并返回请求的承诺;你可以在promise数组上指定然后。如果你使用jQuery而不是你自己的自制解决方案，例如

If you did go the promises way, you would make a map over json, and return the promise of the request; you could then specify a then over the array of promises. If you used jQuery instead of your own homebrew solution, for example,

var requests = json.map(function(d) { return $.getJSON(quoteSearch(d.Symbol), function(j) { quotes.push(j); }); }); $.when(requests).then(function() { res.send(quotes); });

（未测试的代码）。