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    <p>A <strong>closure</strong> is a pair of <strong>a function</strong> and <strong>the environment</strong> in which it was defined (assuming <a href="http://en.wikipedia.org/wiki/Scope_%28programming%29#Lexical_scoping" rel="noreferrer">lexical scoping</a>, which JavaScript uses). Thus, a closure's function can access variables in its environment; if no other function has access to that environment, then all of the variables in it are effectively private and only accessible through the closure's function.</p> <p>The example you provided demonstrates this reasonably well. I've added inline comments to explain the environments.</p> <pre><code>// Outside, we begin in the global environment. function greeter(name, age) { // When greeter is *invoked* and we're running the code here, a new // environment is created. Within this environment, the function's arguments // are bound to the variables `name' and `age'. // Within this environment, another new variable called `message' is created. var message = name + ", who is " + age + " years old, says hi!"; // Within the same environment (the one we're currently executing in), a // function is defined, which creates a new closure that references this // environment. Thus, this function can access the variables `message', `name', // and `age' within this environment, as well as all variables within any // parent environments (which is just the global environment in this example). return function greet() { console.log(message); }; } </code></pre> <p>When <code>var bobGreeter = greeter("Bob", 47);</code> is run, a new closure is created; that is, you've now got a new function instance along with the environment in which it was created. Therefore, your new function has a reference to the `message' variable within said environment, although no one else does.</p> <p>Extra reading: <a href="http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-21.html#%_sec_3.2" rel="noreferrer">SICP Ch 3.2</a>. Although it focuses on Scheme, the ideas are the same. If you understand this chapter well, you'll have a good foundation of how environments and lexical scoping work.</p> <p>Mozilla also has a page dedicated to <a href="https://developer.mozilla.org/en/JavaScript/Guide/Closures" rel="noreferrer">explaining closures</a>.</p>
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    1. CO+1 very detailed answer
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    2. CO+1 for linking to [Mozilla's Closure page](https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Closures) - it's the best, most concise explanation of Closure basics and practical applications that I've found. Highly recommended
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