用于将回调放置在nodej中的语法和方法 [英] syntax and methods for placing callbacks in nodejs

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，

1. 如何在 forEach res.send（quotes） $ c>循环完成。


2. 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。

1. How can I use res.send(quotes) after the forEach loop is complete.
2. 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）。

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