Node.js - 等待多个异步调用 | 等待多个异步调用

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问题描述

我正在尝试在渲染Jade模板之前创建多个MongoDB查询，但我无法弄清楚如何在渲染模板之前等待所有Mongo查询完成。

I'm trying to make multiple MongoDB queries before I render a Jade template, but I can't quite figure out how to wait until all the Mongo Queries are completed before rendering the template.

exports.init = function(req, res){


    var NYLakes = {};
    var NJLakes = {};
    var filterNY = {"State" : "NY"};
    db.collection('lakes').find(filterNY).toArray(function(err, result) {
        if (err) throw err;
        NYLakes = result;
    });

    var filterNJ = {"State" : "NJ"};
    db.collection('lakes').find(filterNJ).toArray(function(err, result) {
        if (err) throw err;
        NJLakes = result;
    });

    res.render('explore/index', {
            NYlakes: NYLakes,
            NJlakes: NJLakes
    });

};

推荐答案

我是下划线/ lodash的粉丝，所以我通常在之后使用 _。这会创建一个只在被调用一定次数后执行的函数。

I'm a big fan of underscore/lodash, so I usually use _.after, which creates a function that only executes after being called a certain number of times.

var finished = _.after(2, doRender);

asyncMethod1(data, function(err){
  //...
  finished();
});

asyncMethod2(data, function(err){
  //...
  finished();
})

function doRender(){
  res.render(); // etc
}

因为javascript提升了用<$ c定义的函数的定义$ c> function funcName（）语法，你的代码自然读取：从上到下。

Since javascript hoists the definition of functions defined with the function funcName() syntax, your code reads naturally: top-to-bottom.

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