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    <p>The difference between <code>(1,eval)</code> and plain old <code>eval</code> is that the former is a <em>value</em> and the latter is an lvalue. It would be more obvious if it were some other identifier:</p> <pre><code>var x; x = 1; (1, x) = 1; // syntax error, of course! </code></pre> <p>That is <code>(1,eval)</code> is an expression that yields <code>eval</code> (just as say, <code>(true &amp;&amp; eval)</code> or <code>(0 ? 0 : eval)</code> would), but it's not a reference to <code>eval</code>.</p> <p>Why do you care?</p> <p>Well, the Ecma spec considers a <em>reference</em> to <code>eval</code> to be a "direct eval call", but an expression that merely yields <code>eval</code> to be an indirect one -- and indirect eval calls are guaranteed to execute in global scope.</p> <p>Things I still don't know:</p> <ol> <li>Under what circumstance does a direct eval call <em>not</em> execute in global scope?</li> <li>Under what circumstance can the <code>this</code> of a function at global scope <em>not</em> yield the global object?</li> </ol> <p>Some more information can be gleaned <a href="http://perfectionkills.com/global-eval-what-are-the-options/">here</a>.</p> <p><strong>EDIT</strong> </p> <p>Apparently, the answer to my first question is, "almost always". A direct <code>eval</code> executes from the <em>current</em> scope. Consider the following code:</p> <pre><code>var x = 'outer'; (function() { var x = 'inner'; eval('console.log("direct call: " + x)'); (1,eval)('console.log("indirect call: " + x)'); })(); </code></pre> <p>Not surprisingly (heh-heh), this prints out:</p> <pre><code>direct call: inner indirect call: outer </code></pre> <p><strong>EDIT</strong></p> <p>After more experimentation, I'm going to provisionally say that <code>this</code> cannot be set to <code>null</code> or <code>undefined</code>. It can be set to other falsy values (0, '', NaN, false), but only <em>very</em> deliberately.</p> <p>I'm going to say your source is suffering from a mild and reversible cranio-rectal inversion and might want to consider spending a week programming in Haskell.</p>
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    1. COWow, didn't know the whole `value` vs `lvalue` thing (well, in practice maybe, but not in words). Nor the ES5 eval rules (not that I should reasonably need to use `eval` ever). Thanks!
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    2. COYeah, `eval` has a lot of nasty sharp edges and should only be used as a last resort and then, very, very carefully.
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    3. COI only once came across a valid use - evaluating a script tag that had been added to the DOM via `innerHtml`
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