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    <p>Simple answer: if value of the <code>str</code> is not referenced from anywhere else (and <code>str</code> itself is not referenced from <code>restofprogram</code>) it will become unreachable as soon as the <code>function (str) { ... }</code> returns.</p> <p>Details: V8 compiler distinguishes real <em>local</em> variables from so called <em>context</em> variables captured by a closure, shadowed by a <strong>with</strong>-statement or an <code>eval</code> invocation. </p> <p>Local variables live on the stack and disappear as soon as function execution completes.</p> <p>Context variables live in a heap allocated context structure. They disappear when the context structure dies. Important thing to note here is that context variables from the same scope live in the <strong>same</strong> structure. Let me illustrate it with an example code:</p> <pre><code>function outer () { var x; // real local variable var y; // context variable, referenced by inner1 var z; // context variable, referenced by inner2 function inner1 () { // references context use(y); } function inner2 () { // references context use(z); } function inner3 () { /* I am empty but I still capture context implicitly */ } return [inner1, inner2, inner3]; } </code></pre> <p>In this example variable <code>x</code> will disappear as soon as <code>outer</code> returns but variables <code>y</code> and <code>z</code> will disappear only when <strong>both</strong> <code>inner1</code>, <code>inner2</code> <strong>and</strong> <code>inner3</code> die. This happens because <code>y</code> and <code>z</code> are allocated in the same context structure and all three closures implicitly reference this context structure (even <code>inner3</code> which does not use it explicitly).</p> <p>Situation gets even more complicated when you start using <strong>with</strong>-statement, <strong>try/catch</strong>-statement which on V8 contains an implicit <strong>with</strong>-statement inside catch clause or global <code>eval</code>.</p> <pre><code>function complication () { var x; // context variable function inner () { /* I am empty but I still capture context implicitly */ } try { } catch (e) { /* contains implicit with-statement */ } return inner; } </code></pre> <p>In this example <code>x</code> will disappear only when <code>inner</code> dies. Because: </p> <ul> <li><strong>try/catch</strong>-contains implicit <strong>with</strong>-statement in catch clause</li> <li>V8 assumes that any <strong>with</strong>-statement shadows <em>all</em> the locals </li> </ul> <p>This forces <code>x</code> to become a context variable and <code>inner</code> captures the context so <code>x</code> exists until <code>inner</code> dies.</p> <p>In general if you want to be sure that given variable does not retain some object for longer than really needed you can easily <em>destroy</em> this link by assigning <code>null</code> to that variable.</p>
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    1. COthanks a lot, this is very informative
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