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copied!<p>The first step to understanding what is happening is understanding what's happening behind the scenes. From a language standpoint, there's nothing special about node modules. The 'magic' comes from how node loads files from disk when you <code>require</code>.</p> <p>When you call <code>require</code>, node either synchronously reads from disk or returns the module's cached exports object. When reading files, it follows <a href="http://nodejs.org/api/modules.html#modules_all_together">a set of somewhat complex rules</a> to determine exactly <em>which</em> file is read, but once it has a path:</p> <ol> <li>Check if <code>require.cache[moduleName]</code> exists. If it does, return that and STOP.</li> <li><code>code = fs.readFileSync(path)</code>.</li> <li><a href="https://github.com/joyent/node/blob/v0.10.5/src/node.js#L878-885">Wrap</a> (concatenate) <code>code</code> with the string <code>(function (exports, require, module, __filename, __dirname) {</code> ... <code>});</code></li> <li><p><code>eval</code> your wrapped code and invoke the anonymous wrapper function.</p> <pre><code>var module = { exports: {} }; eval(code)(module.exports, require, module, path, pathMinusFilename); </code></pre></li> <li><p>Save <code>module.exports</code> as <code>require.cache[moduleName]</code>.</p></li> </ol> <p>The next time you <code>require</code> the same module, node simply returns the cached <code>exports</code> object. (This is a very good thing, because the initial loading process is slow and synchronous.)</p> <p>So now you should be able to see:</p> <ul> <li>Top-level code in a module is only executed once.</li> <li>Since it is actually executed in an anonymous function: <ul> <li>'Global' variables aren't actually global (unless you explicitly assign to <code>global</code> or don't scope your variables with <code>var</code>)</li> <li>This is how a module gets a local scope.</li> </ul></li> </ul> <p>In your example, you <code>require</code> module <em>a</em> for each request, <em>but you're actually sharing the same module scope across all requrests</em> because of the module caching mechanism outlined above. Every call to <code>printName</code> shares the same <code>a</code> in its scope chain (even though <code>printName</code> itself gets a new scope on each invocation).</p> <p>Now in the literal code you have in your question, this doesn't matter: you set <code>a</code> and then use it on the very next line. Control never leaves <code>printName</code>, so the fact that <code>a</code> is shared is irrelevant. My guess is your real code looks more like:</p> <pre><code>var a; function printName(req, res) { //get param `name` from url; a = name; getSomethingFromRedis(function(result) { res.end('Hi '+a); }); } module.exports.printName = printName; </code></pre> <p>Here we have a problem because control <em>does</em> leave <code>printName</code>. The callback eventually fires, but another request changed <code>a</code> in the meantime.</p> <p>You probably want something more like this:</p> <p><strong>a.js</strong></p> <pre><code>module.exports = function A() { var a; function printName(req, res) { //get param `name` from url; a = name; res.end('Hi '+a); } return { printName: printName }; } </code></pre> <p><strong>index.js</strong></p> <pre><code>var A = require('a'); function requestListener(req, res) { var a = A(); a.printName(req, res); } </code></pre> <p>This way, you get a fresh and independent scope inside of <code>A</code> for each request.</p>

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