blocks|key|292079|text|基本上不是闭包是如何工作的，函数继承了它们的外部作用域，这就是它的工作方式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|292080|//+this+function+only+inherits+the+global+scope
function+writeBody()
{
++++res.end("<h1>+Hooray!+</h1>");
}

http.createServer(function(req,+res)+//+a+new+local+varaible+res+is+created+here+for+each+callback
{
++++res.writeHead('Content-Type',+'text/html');
++++//+annonymous+function+inheris+both+the+global+scope
++++//+as+well+as+the+scope+of+the+server+callback
++++setTimeout(function(){

++++++++//+the+local+variable+res+is+available+here+too
++++++++writeBody()

++++},+2000);
}).listen(8000);|code-block|syntax|javascript|292081|要使其工作，只需将res对象传递到函数中，因为它在超时回调中可用。|offset|length|style|CODE|292082|function+writeBody(res)
{
++++//+NOT+the+same+variable+res,+but+it+holds+the+same+value
++++res.end("<h1>+Hooray!+</h1>");
}

http.createServer(function(req,+res)
{
++++res.writeHead('Content-Type',+'text/html');
++++setTimeout(function(){
++++++++writeBody(res);+//+just+pass+res
++++},+2000);
}).listen(8000);|292083|但是你需要注意这样的事情：|292084|for(var+i+=+0;+i+<+10;+i%2B%2B)+{+//+only+one+i+gets+created+here!()
++++setTimeout(function()+{
++++++++console.log(i);+//+this+always+references+the+same+variable+i
++++},+1000);
}|292085|这将打印10十次，因为引用是相同的，并且i会一直递增到10。如果你想拥有不同的数字，你需要为每个数字创建一个新的变量，方法是将setTimeout封装到匿名的self函数中，并将其作为参数传递给i，或者调用一些其他方法来设置timouet并将i作为参数接收。|292086|//+anoynmous+function+version
for(var+i+=+0;+i+<+10;+i%2B%2B)+{
++++(function(e){+//+creates+a+new+variable+e+for+each+call
++++++++setTimeout(function()+{
++++++++++++console.log(e);+
++++++++},+1000);
++++})(i);+//+pass+in+the+value+of+i
}

//+function+call+version
for(var+i+=+0;+i+<+10;+i%2B%2B)+{
++++createTimeoutFunction(i);
}|292087|entityMap^0|0|0|9|3|0|0|0|0|4|2|K|1|R|2|1R|A|2P|1|3D|1|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|14|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|16|8|@$I|17|J|18|K|L]|$I|19|J|1A|K|L]|$I|1B|J|1C|K|L]|$I|1D|J|1E|K|L]|$I|1F|J|1G|K|L]|$I|1H|J|1I|K|L]]|9|@]|A|$]]|$1|U|3|V|5|D|7|1J|8|@]|9|@]|A|$E|F]]|$1|W|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|X|$]]

Basically that not how closures work, functions inherit their outer scopes that's how it works.

<pre><code>// this function only inherits the global scope
function writeBody()
{
 res.end("&lt;h1&gt; Hooray! &lt;/h1&gt;");
}

http.createServer(function(req, res) // a new local varaible res is created here for each callback
{
 res.writeHead('Content-Type', 'text/html');
 // annonymous function inheris both the global scope
 // as well as the scope of the server callback
 setTimeout(function(){

 // the local variable res is available here too
 writeBody()

 }, 2000);
}).listen(8000);
</code></pre>

To make it work just pass the <code>res</code> object into the function, as it's available in the timeout callback.

<pre><code>function writeBody(res)
{
 // NOT the same variable res, but it holds the same value
 res.end("&lt;h1&gt; Hooray! &lt;/h1&gt;");
}

http.createServer(function(req, res)
{
 res.writeHead('Content-Type', 'text/html');
 setTimeout(function(){
 writeBody(res); // just pass res
 }, 2000);
}).listen(8000);
</code></pre>

But you need to watch out for things like this: 

<pre><code>for(var i = 0; i &lt; 10; i++) { // only one i gets created here!()
 setTimeout(function() {
 console.log(i); // this always references the same variable i
 }, 1000);
}
</code></pre>

This will print <code>10</code> ten times, because the reference is the same and <code>i</code> gets incremented all the way up to <code>10</code>. If you want to have the different numbers you need to create a new variable for each one, either by wrapping the <code>setTimeout</code> into an anonymous self function which you pass in the <code>i</code> as a parameter, or by calling some other method which sets up the timouet and receives the <code>i</code> as a parameter.

<pre><code>// anoynmous function version
for(var i = 0; i &lt; 10; i++) {
 (function(e){ // creates a new variable e for each call
 setTimeout(function() {
 console.log(e); 
 }, 1000);
 })(i); // pass in the value of i
}

// function call version
for(var i = 0; i &lt; 10; i++) {
 createTimeoutFunction(i);
}
</code></pre>

blocks|key|3006620|text|您还可以使函数嵌套，以便它们共享作用域，即|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3006621|http.createServer(function(req,+res)
{

++++function+writeBody()
++++{
++++++++res.end("<h1>+Hooray!+</h1>");
++++}

++++res.writeHead('Content-Type',+'text/html');
++++setTimeout(function(){writeBody()},+2000);
}).listen(8000);|code-block|syntax|javascript|3006622|我经常发现这比总是传递一堆变量来保持作用域更容易，尽管这意味着你不能在其他地方重用函数。|3006623|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You can also make the functions nested, so they share scope, ie

<pre><code>http.createServer(function(req, res)
{

 function writeBody()
 {
 res.end("&lt;h1&gt; Hooray! &lt;/h1&gt;");
 }

 res.writeHead('Content-Type', 'text/html');
 setTimeout(function(){writeBody()}, 2000);
}).listen(8000);
</code></pre>

I often find this is easier than always passing a bunch of variables in to keep in scope, although it means you cannot reuse the function elsewhere.

blocks|key|3006640|text|你可以在你的回调中传递响应，这样：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3006641|http.createServer(function(req,+res)
{
++++res.writeHead('Content-Type',+'text/html');
++++setTimeout(function(){writeBody(res)},+2000);
}).listen(8000);|code-block|syntax|javascript|3006642|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can pass response in your callback so:

<pre><code>http.createServer(function(req, res)
{
 res.writeHead('Content-Type', 'text/html');
 setTimeout(function(){writeBody(res)}, 2000);
}).listen(8000);
</code></pre>

blocks|key|289764|text|我真的很喜欢Justin+Cormack的回答。这是我最近的一些编码的一个更极端的例子。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|289765|var+Func4+=+function(req,+res)
{
++var+collectionName+=+"parts";
++var+f0+=+function()+{mongodbClient.collection(collectionName,+f1);};
++var+f1+=+function(err,+coll)+{coll.ensureIndex("item",+f2);};
++var+f2+=+function(err,+indexname)
++{
++++res.writeHead(200,+{'Content-Type':+'text/plain'});
++++res.write("index+name+=+"+%2B+indexname);
++++res.end();
++};
++f0();
};|code-block|syntax|javascript|289766|大多数人会告诉我(他们确实这样做了)，这是编写代码的正确方式。|289767|var+Func4+=+function(req,+res)
{
++var+collectionName+=+"parts";
++mongodbClient.collection(collectionName,+function(err,+coll)+{
++++coll.ensureIndex("item",+function(err,+indexname)+{
++++++res.writeHead(200,+{'Content-Type':+'text/plain'});
++++++res.write("index+name+=+"+%2B+indexname);
++++++res.end();
++++})});
};|289768|也许我是一个n00b，但我发现嵌套回调有点难以理解。我也承认一些f0、f1、f2函数很差劲。无论哪种方式，这都是一个很好的作用域示例。|289769|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I actually like Justin Cormack's answer. Here is a more extreme example of some recent coding of mine. 

<pre><code>var Func4 = function(req, res)
{
 var collectionName = "parts";
 var f0 = function() {mongodbClient.collection(collectionName, f1);};
 var f1 = function(err, coll) {coll.ensureIndex("item", f2);};
 var f2 = function(err, indexname)
 {
 res.writeHead(200, {'Content-Type': 'text/plain'});
 res.write("index name = " + indexname);
 res.end();
 };
 f0();
};
</code></pre>

Most people would tell me (and they do) that this is the correct way to write that code.

<pre><code>var Func4 = function(req, res)
{
 var collectionName = "parts";
 mongodbClient.collection(collectionName, function(err, coll) {
 coll.ensureIndex("item", function(err, indexname) {
 res.writeHead(200, {'Content-Type': 'text/plain'});
 res.write("index name = " + indexname);
 res.end();
 })});
};
</code></pre>

Perhaps I am a n00b, but I find nested callbacks a bit hard to follow. I also admit a bunch of f0,f1,f2 functions is lame. Either way, this is a good example of scope.

I'm an experienced software developer, but pretty new to JS and to node. I'm not a big fan of super-nested code, so I've been trying to break callbacks out into their own functions. I'm having trouble though with figuring out how to maintain scope when the callback fires. Digging around I read that if I created a closure over the callback it would work, but it doesn't seem to work the way I expected it would.

Here's a very simple version of the kind of code that isn't working for me:

<pre><code>function writeBody()
{
 res.end("&lt;h1&gt; Hooray! &lt;/h1&gt;");
}

http.createServer(function(req, res)
{
 res.writeHead('Content-Type', 'text/html');
 setTimeout(function(){writeBody()}, 2000);
}).listen(8000);
</code></pre>

I thought that by wrapping the writeBody() call in the function() closure I would have the scope I needed after the timeout, but when writeBody() fires I get 

<blockquote>
 ReferenceError: res is not defined
</blockquote>

Can anyone tell me what boneheaded thing I am doing wrong?

How can I maintain scope in node.js callbacks?

软件开发

我是一名经验丰富的软件开发人员，但对于JS和node来说，我还是个新手。我不是超级嵌套代码的狂热爱好者，所以我一直在尝试将回调分解到它们自己的函数中。不过，我在弄清楚如何在回调触发时维护作用域时遇到了问题。深入研究后，我读到如果我在回调上创建一个闭包，它将会工作，但它似乎不会以我预期的方式工作。这是一个非常简单的代码版...

问如何在node.js回调中维护作用域？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何在node.js回调中维护作用域？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何在node.js回调中维护作用域？
EN