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copied!<p><strong>Editor's Note:</strong> All functions in JavaScript are closures as explained in this <a href="http://javascriptweblog.wordpress.com/2010/10/25/understanding-javascript-closures/" rel="noreferrer" title="Understanding JavaScript Closures | JavaScript, JavaScript…">post</a>. However we are only interested in identifying a subset of these functions which are <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Closures" rel="noreferrer" title="Closures - JavaScript | MDN">interesting</a> from a theoretical point of view. Henceforth any reference to the word <em>closure</em> will refer to this subset of functions unless otherwise stated.</p> <p>A simple explanation for closures:</p> <ol> <li>Take a function. Let's call it F.</li> <li>List all the variables of F.</li> <li>The variables may be of two types: <ol> <li>Local variables (bound variables)</li> <li>Non-local variables (free variables)</li> </ol></li> <li>If F has no free variables then it cannot be a closure.</li> <li>If F has any free variables (which are defined in <strong>a</strong> parent scope of F) then: <ol> <li>There must be only one parent scope of F to which <strong>a</strong> free variable is bound.</li> <li>If F is <strong>referenced</strong> from outside <strong>that</strong> parent scope, then it becomes a closure for <strong>that</strong> free variable.</li> <li><strong>That</strong> free variable is called an upvalue of the closure F.</li> </ol></li> </ol> <p>Now let's use this to figure out who uses closures and who doesn't (for the sake of explanation I have named the functions):</p> <p><strong>Case 1: Your Friend's Program</strong></p> <pre><code>for (var i = 0; i < 10; i++) { (function f() { var i2 = i; setTimeout(function g() { console.log(i2); }, 1000); })(); } </code></pre> <p>In the above program there are two functions: <code>f</code> and <code>g</code>. Let's see if they are closures:</p> <p>For <code>f</code>:</p> <ol> <li>List the variables: <ol> <li><code>i2</code> is a <strong>local</strong> variable.</li> <li><code>i</code> is a <strong>free</strong> variable.</li> <li><code>setTimeout</code> is a <strong>free</strong> variable.</li> <li><code>g</code> is a <strong>local</strong> variable.</li> <li><code>console</code> is a <strong>free</strong> variable.</li> </ol></li> <li>Find the parent scope to which each free variable is bound: <ol> <li><code>i</code> is <strong>bound</strong> to the global scope.</li> <li><code>setTimeout</code> is <strong>bound</strong> to the global scope.</li> <li><code>console</code> is <strong>bound</strong> to the global scope.</li> </ol></li> <li>In which scope is the function <strong>referenced</strong>? The <strong>global scope</strong>. <ol> <li>Hence <code>i</code> is not <strong>closed over</strong> by <code>f</code>.</li> <li>Hence <code>setTimeout</code> is not <strong>closed over</strong> by <code>f</code>.</li> <li>Hence <code>console</code> is not <strong>closed over</strong> by <code>f</code>.</li> </ol></li> </ol> <p>Thus the function <code>f</code> is not a closure.</p> <p>For <code>g</code>:</p> <ol> <li>List the variables: <ol> <li><code>console</code> is a <strong>free</strong> variable.</li> <li><code>i2</code> is a <strong>free</strong> variable.</li> </ol></li> <li>Find the parent scope to which each free variable is bound: <ol> <li><code>console</code> is <strong>bound</strong> to the global scope.</li> <li><code>i2</code> is <strong>bound</strong> to the scope of <code>f</code>.</li> </ol></li> <li>In which scope is the function <strong>referenced</strong>? The <strong>scope of <code>setTimeout</code></strong>. <ol> <li>Hence <code>console</code> is not <strong>closed over</strong> by <code>g</code>.</li> <li>Hence <code>i2</code> is <strong>closed over</strong> by <code>g</code>.</li> </ol></li> </ol> <p>Thus the function <code>g</code> is a closure for the free variable <code>i2</code> (which is an upvalue for <code>g</code>) <strong>when</strong> it's <strong>referenced</strong> from within <code>setTimeout</code>.</p> <p><em>Bad for you:</em> Your friend is using a closure. The inner function is a closure.</p> <p><strong>Case 2: Your Program</strong></p> <pre><code>for (var i = 0; i < 10; i++) { setTimeout((function f(i2) { return function g() { console.log(i2); }; })(i), 1000); } </code></pre> <p>In the above program there are two functions: <code>f</code> and <code>g</code>. Let's see if they are closures:</p> <p>For <code>f</code>:</p> <ol> <li>List the variables: <ol> <li><code>i2</code> is a <strong>local</strong> variable.</li> <li><code>g</code> is a <strong>local</strong> variable.</li> <li><code>console</code> is a <strong>free</strong> variable.</li> </ol></li> <li>Find the parent scope to which each free variable is bound: <ol> <li><code>console</code> is <strong>bound</strong> to the global scope.</li> </ol></li> <li>In which scope is the function <strong>referenced</strong>? The <strong>global scope</strong>. <ol> <li>Hence <code>console</code> is not <strong>closed over</strong> by <code>f</code>.</li> </ol></li> </ol> <p>Thus the function <code>f</code> is not a closure.</p> <p>For <code>g</code>:</p> <ol> <li>List the variables: <ol> <li><code>console</code> is a <strong>free</strong> variable.</li> <li><code>i2</code> is a <strong>free</strong> variable.</li> </ol></li> <li>Find the parent scope to which each free variable is bound: <ol> <li><code>console</code> is <strong>bound</strong> to the global scope.</li> <li><code>i2</code> is <strong>bound</strong> to the scope of <code>f</code>.</li> </ol></li> <li>In which scope is the function <strong>referenced</strong>? The <strong>scope of <code>setTimeout</code></strong>. <ol> <li>Hence <code>console</code> is not <strong>closed over</strong> by <code>g</code>.</li> <li>Hence <code>i2</code> is <strong>closed over</strong> by <code>g</code>.</li> </ol></li> </ol> <p>Thus the function <code>g</code> is a closure for the free variable <code>i2</code> (which is an upvalue for <code>g</code>) <strong>when</strong> it's <strong>referenced</strong> from within <code>setTimeout</code>.</p> <p><em>Good for you:</em> You are using a closure. The inner function is a closure.</p> <p>So both you and your friend are using closures. Stop arguing. I hope I cleared the concept of closures and how to identify them for the both of you.</p> <p><strong>Edit:</strong> A simple explanation as to why are all functions closures (credits @Peter):</p> <p>First let's consider the following program (it's the <a href="http://jsfiddle.net/KyQKw/" rel="noreferrer">control</a>):</p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>lexicalScope(); function lexicalScope() { var message = "This is the control. You should be able to see this message being alerted."; regularFunction(); function regularFunction() { alert(eval("message")); } }</code></pre> </div> </div> </p> <ol> <li>We know that both <code>lexicalScope</code> and <code>regularFunction</code> aren't closures <strong>from the above definition</strong>.</li> <li>When we execute the program <strong>we expect</strong> <code>message</code> to be alerted <strong>because</strong> <code>regularFunction</code> is not a closure (i.e. it has access to <strong>all</strong> the variables in its parent scope - including <code>message</code>).</li> <li>When we execute the program <strong>we observe</strong> that <code>message</code> is indeed alerted.</li> </ol> <p>Next let's consider the following program (it's the <a href="http://jsfiddle.net/KyQKw/1/" rel="noreferrer">alternative</a>):</p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>var closureFunction = lexicalScope(); closureFunction(); function lexicalScope() { var message = "This is the alternative. If you see this message being alerted then in means that every function in JavaScript is a closure."; return function closureFunction() { alert(eval("message")); }; }</code></pre> </div> </div> </p> <ol> <li>We know that only <code>closureFunction</code> is a closure <strong>from the above definition</strong>.</li> <li>When we execute the program <strong>we expect</strong> <code>message</code> not to be alerted <strong>because</strong> <code>closureFunction</code> is a closure (i.e. it only has access to all its <strong>non-local variables</strong> at <strong>the time the function is created</strong> (<a href="https://stackoverflow.com/a/12930799/783743" title="scope - JavaScript closures vs. anonymous functions - Stack Overflow">see this answer</a>) - this does not include <code>message</code>).</li> <li>When we execute the program <strong>we observe</strong> that <code>message</code> is actually being alerted.</li> </ol> <p>What do we infer from this?</p> <ol> <li>JavaScript interpreters do not treat closures differently from the way they treat other functions.</li> <li>Every function carries its <a href="http://dmitrysoshnikov.com/ecmascript/chapter-4-scope-chain/" rel="noreferrer" title="ECMA-262 » ECMA-262-3 in detail. Chapter 4. Scope chain.">scope chain</a> along with it. Closures don't have a <strong>separate</strong> referencing environment.</li> <li>A closure is just like every other function. We just call them closures when they are <strong>referenced</strong> in a scope <strong>outside</strong> the scope to which they belong <strong>because</strong> this is an interesting case.</li> </ol>

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