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POIndirect eval call in strict mode
 

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  1. POIndirect eval call in strict mode
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    <p>I understand about how <code>eval()</code> works in non-strict contexts, however the case of using <code>eval()</code> in strict mode has completely befuddled me. When <code>eval()</code> is called directly in the global scope, variables are kept inside the new <code>eval()</code> scope:</p> <pre><code>'use strict'; eval('var a = 1;'); console.log(a); // ReferenceError: a is not defined </code></pre> <p>However, if I perform an <em>indirect</em> call to <code>eval()</code> in the global scope (should be the same thing, right?), it acts as though it is not in strict mode (if you don't believe me, see <a href="http://jsfiddle.net/hSLJq/3/" rel="nofollow noreferrer">this JSFiddle</a>):</p> <pre><code>'use strict'; (0, eval)('var a = 1;'); // indirect call to eval console.log(a); // 1??? </code></pre> <p><sub><em>If you don't understand what <code>(0, eval)</code> does, see <a href="https://stackoverflow.com/questions/19535601/why-does-google-main-page-use-0-obj-funcargs-syntax">Why does google main page use (0, obj.func)(args) syntax?</a>.</em></sub></p> <p>At least according to my understanding of how <code>eval()</code> is supposed to work in strict mode, it is meant to (no matter whether <code>eval()</code> is called directly or indirectly) create a new scope for variables defined in the <code>eval()</code> call, however this doesn't seem to be the case here. The specification says the following:</p> <blockquote> <h3><a href="http://ecma-international.org/ecma-262/5.1/#sec-10.4.2" rel="nofollow noreferrer" title="link to this section">10.4.2</a> Entering Eval Code</h3> <p>The following steps are performed when control enters the execution context for eval code:</p> <ol> <li><p>If there is no calling context or if the eval code is not being evaluated by a direct call (<a href="http://ecma-international.org/ecma-262/5.1/#sec-15.1.2.1.1" rel="nofollow noreferrer">15.1.2.1.1</a>) to the eval function then,</p> <p>a. Initialise the execution context as if it was a global execution context using the eval code as <em>C</em> as described in <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.4.1.1" rel="nofollow noreferrer">10.4.1.1</a>.</p></li> <li><p>Else, </p> <p>a. Set the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">ThisBinding</a> to the same value as the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">ThisBinding</a> of the calling execution context.<br> b. Set the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">LexicalEnvironment</a> to the same value as the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">LexicalEnvironment</a> of the calling execution context.<br> c. Set the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">VariableEnvironment</a> to the same value as the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">VariableEnvironment</a> of the calling execution context.</p></li> <li><p>If the eval code is <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.1.1" rel="nofollow noreferrer">strict code</a>, then</p> <p>a. Let <em>strictVarEnv</em> be the result of calling <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.2.2.2" rel="nofollow noreferrer">NewDeclarativeEnvironment</a> passing the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">LexicalEnvironment</a> as the argument.<br> b. Set the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">LexicalEnvironment</a> to <em>strictVarEnv</em>.<br> c. Set the <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.3" rel="nofollow noreferrer">VariableEnvironment</a> to <em>strictVarEnv</em>.</p></li> <li><p>Perform <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.5" rel="nofollow noreferrer">Declaration Binding Instantiation</a> as described in <a href="http://ecma-international.org/ecma-262/5.1/#sec-10.5" rel="nofollow noreferrer">10.5</a> using the eval code.</p></li> </ol> </blockquote> <p>This is the case in all major browsers, including (but not limited to) Internet Explorer 10, Chrome 30 and Firefox 24 - as all of them have the same behaviour, I don't think it's likely that it's a bug. Aren't they both meant to do the same thing, and if not, why is this the case?</p> <p><sub>Note: <em>please</em> don't tell me to not use <code>eval()</code> (yes, I know the "dangers" of using <code>eval()</code>) - I simply want to understand the logic behind this, which is utterly confusing to me.</sub></p>
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    1. COFeels like a bug to me. How did you execute the code above? Which browser/JS interpreter?
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    2. CO@AaronDigulla: in the JSFiddle given, Chrome 30.
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    3. CO@AaronDigulla: Firefox 24 and IE10 also exhibits this behaviour.
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