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POJS Function-constructor re-parsed everytime?
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copied!<p>In MDN, about <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope#Function_constructor_vs._function_declaration_vs._function_expression" rel="nofollow">Functions and function scope</a>, what does it mean by <strong>parsed every time it is evaluated</strong>? Can this be observed by codes?</p> <p>Quote in the section <strong>Function constructor vs. function declaration vs. function expression</strong>:</p> <blockquote> <p>Functions defined by function expressions and function declarations are <strong>parsed only once</strong>, while those defined by the Function constructor <strong>are not</strong>. That is, the function body string passed to the Function constructor must be <strong>parsed every time it is evaluated</strong>. Although a function expression creates a closure every time, the function body is <strong>not reparsed</strong>, so function expressions are still faster than "new Function(...)". Therefore the Function constructor should be avoided whenever possible.<p> It should be noted, however, that function expressions and function declarations nested within the function generated by parsing a Function constructor 's string <strong>aren't parsed repeatedly</strong>. For example:<p><code>var foo = (new Function("var bar = \'FOO!\';\nreturn(function() {\n\talert(bar);\n});"))();<br/> foo(); //The segment "function() {\n\talert(bar);\n}" of the function body string is not re-parsed.</code></p> </blockquote> <p>I have written a code snippet to (try to) test and understand it:</p> <pre><code>var bar = 'FOO!'; var foo = (new Function("return(function() {\n\talert(bar);\n});"))(); bar = 'FOO! again'; foo(); //The segment "function() {\n\talert(bar);\n}" of the function body string is not re-parsed. var bar2 = 'FOO!2'; var foo2 = function() { alert(bar2); }; bar2 = 'FOO!2 again'; foo2(); </code></pre> <p>Both two alerts the "again-version".</p> <p>What does it mean by <strong>reparsed</strong> or not?</p> <p>Can this be illustrated by code results?</p> <p>Thanks.</p> <hr> <p>FYI, i have tried another code snippet:</p> <pre><code>var bar = 'FOO!'; var string1 = "return(function() {\n\talert(bar);\n});"; var foo = (new Function(string1))(); bar = 'FOO! again'; foo(); //The segment "function() {\n\talert(bar);\n}" of the function body string is not re-parsed. string1 = "return(function() {\n\talert(bar + ' more');\n});"; foo(); </code></pre> <p>Both alerts "FOO! again", <strong><em>not</em></strong> "FOO! again more".</p>

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