blocks|key|2619158|text|cursor.each()是异步的。这意味着它有时会在你的res.send(str)之后运行，因此你会得到以前版本的str。您需要首先收集所有数据，然后仅当您获得所有数据时才发送响应。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2619159|如果您想要所有数据，那么可以使用promises和.toArray()，如下所示：|2619160|app.route('/view-reports').get(function(req,+res)+{
++collections.find({}).toArray().then(data+=>+{
++++++let+result+=+data.map(item+=>+{
++++++++++return+"Substance:+"+%2B+item.substance+%2B+"<br>";
++++++}).join("");
++++++res.send(result);
++}).catch(err+=>+{
++++++//+database+error
++++++console.log(err);
++++++res.sendStatus(500);
++});
});|code-block|syntax|javascript|2619161|注意:这也明智地去掉了str变量，该变量超出了请求的范围，因此当多个请求同时运行(来自不同用户)时，很容易导致并发错误。|2619162|entityMap^0|0|D|T|D|1M|3|0|P|A|0|0|B|3|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|P|$]]

<p><code>cursor.each()</code> is asynchronous.  That means it runs sometimes LATER, after your <code>res.send(str)</code>, thus you get the previous version of <code>str</code>.  You need to collect all the data first and then send your response only when you have all the data.</p>

<p>If you want all the data, then you could use promises and <code>.toArray()</code> like this:</p>

<pre><code>app.route('/view-reports').get(function(req, res) {
  collections.find({}).toArray().then(data =&gt; {
      let result = data.map(item =&gt; {
          return "Substance: " + item.substance + "&lt;br&gt;";
      }).join("");
      res.send(result);
  }).catch(err =&gt; {
      // database error
      console.log(err);
      res.sendStatus(500);
  });
});
</code></pre>

<p>Note: This also wisely gets rid of the <code>str</code> variable which was outside the scope of the request and thus could easily lead to a concurrency bug when multiple requests were in flight at the same time (from different users).</p>


blocks|key|2619197|text|创建一个专门针对物质的路由器，并在app中使用它。您可以创建一个ul，而不是中断，而且，处理应该在前端进行。分离你们的关注点。服务器不应该担心任何渲染等问题。每个进程只有一个目的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2619198|可以为每个资源创建路由器。为猫和狗创建一个物质路由器。每个单独的路由器都有自己的get、post、delete和puts，允许您修改该资源。app可以同时使用所有路由器。|offset|length|style|CODE|2619199|app.use(catRouter);+app.use(mooseRouter);+app.use(platypusRouter);|2619200|​|2619201|const+{+Router+}+=+require('express');
const+createError+=+require('http-errors');
let+substanceRouter+=+new+Router();

function+buildElement(arr)
{
++let+start+=+'';
++arr.forEach(val+=>+{
++++if(!val)+return;
++++start+%2B=+`Substance+:+${val}<br>`;
++});
++return+start;
}

subtanceRouter.get('/endpoint/whatever',+function(req,+res,+next)+{

++collectios.find({})
++
++.then(results+=>+{
++++if(!results)+throw+new+Error('Resource+Not+Found');
++++
++++let+output+=+buildElement(results);
++++res.json(output);
++++next();
++})
++
++.catch(err+=>+next(createError(404,+err.message)));
})

app.use(substanceRouter);|code-block|syntax|javascript|2619202|2619203|或者，我们可以这样写：|2619204|2619205|let+output+=+results
++++++++++++.filter(sub+=>+!!sub)
++++++++++++.join('<br>');
res.json(output);|2619206|2619207|但需要注意的是，这将增加内存开销，生成一个全新的数组来存放结果，最坏情况下会消耗O(n)内存。|2619208|entityMap^0|0|1Y|3|0|0|1U|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|12|8|@$D|13|E|14|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|15|8|@$D|16|E|17|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|18|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|19|8|@]|9|@]|A|$O|P]]|$1|Q|3|K|5|6|7|1A|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|1B|8|@]|9|@]|A|$]]|$1|T|3|K|5|6|7|1C|8|@]|9|@]|A|$]]|$1|U|3|V|5|N|7|1D|8|@]|9|@]|A|$O|P]]|$1|W|3|K|5|6|7|1E|8|@]|9|@]|A|$]]|$1|X|3|Y|5|6|7|1F|8|@]|9|@]|A|$]]|$1|Z|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]]|10|$]]

<p>Create a router specifically for substances and use it in app. Instead of breaks, you can create a ul, also, that processing should happen on the front end. Separate your concerns. The server shouldn't have to worry about any rendering and etc. One purpose per process.</p>

<p>The routers can be created per resource. Create a router for substances, for cats, for dogs. Each individual router has it's own get post delete and puts that allow you to modify that resource. <code>app</code> can use all the routers at once.</p>

<p><code>app.use(catRouter);
app.use(mooseRouter);
app.use(platypusRouter);</code></p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>const { Router } = require('express');
const createError = require('http-errors');
let substanceRouter = new Router();

function buildElement(arr)
{
  let start = '';
  arr.forEach(val =&gt; {
    if(!val) return;
    start += `Substance : ${val}&lt;br&gt;`;
  });
  return start;
}

subtanceRouter.get('/endpoint/whatever', function(req, res, next) {

  collectios.find({})
  
  .then(results =&gt; {
    if(!results) throw new Error('Resource Not Found');
    
    let output = buildElement(results);
    res.json(output);
    next();
  })
  
  .catch(err =&gt; next(createError(404, err.message)));
})

app.use(substanceRouter);</code></pre>
</div>
</div>
</p>

<p>Alternately we can write :</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>let output = results
            .filter(sub =&gt; !!sub)
            .join('&lt;br&gt;');
res.json(output);</code></pre>
</div>
</div>
</p>

<p>But be advised this will add memory overhead, generates a completely new array to house the results, consuming at worst, O(n) memory.</p>


<p>I'm building an application using Node/Express/MongoDB (first time with all these) that will let me pull data from the DB and display it on an Express page. This is what the GET request looks like:</p>
<pre><code>var str = &quot;&quot;;

app.route('/view-reports').get(function(req, res) {
  var cursor = collections.find({});

  cursor.each(function(err, item) {
    if (item != null) {
      console.log(str);
      str = str + &quot;Substance: &quot; + item.substance + &quot;&lt;br&gt;&quot;;
    }
    if (err) {
      console.log(err);
    }
  });
  console.log(str);
  res.send(str);
  str = &quot;&quot;;
});
</code></pre>
<p>I would expect this to return something like this:</p>
<blockquote>
<p>Substance: a</p>
<p>Substance: b</p>
<p>Substance: c</p>
</blockquote>
<p>However, the initial request does not return anything at all. The second request will return the above. If I enclose <code>res.send(str)</code> in an if conditional it simply will not load until a second request is made.</p>


Express.js GET request not returning data on first call

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我正在使用Node/ Express /MongoDB构建一个应用程序(这是我第一次使用所有这些工具)，它允许我从数据库中提取数据并将其显示在Express页面上。下面是GET请求的样子：var str = "";app.route('/view-reports').get(function(req, res) {  ...

问Express.js GET请求在第一次调用时未返回数据
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