Why is "this" in JavaScript said to be an implicit parameter?
<p>I wonder why in the book <a href="http://rads.stackoverflow.com/amzn/click/193398869X" rel="nofollow">Secrets of the JavaScript Ninja by John Resig</a>, p. 48, it is said that:</p>

<blockquote>
  <p>Whenever a function is invoked, ... an implicit parameter named <code>this</code>
  is also passed to the function.</p>
</blockquote>

<p>I was a little puzzled because previously, I read that <code>this</code> is actually a keyword.  And maybe it doesn't really matter, except if we try</p>

<pre><code>function f() {
    this = {};
}

f();
</code></pre>

<p>then either Chrome or Node.js will raise an error that it is a invalid left side in assignment.  So if <code>this</code> is actually an implicit parameter, then that line shouldn't raise an error?  So I wonder is it true that the book has this mistake about <code>this</code> and it should be otherwise?</p>

<p>(<strong>Update:</strong> I also re-checked JavaScript: the Definitive Guide 6th Edition and ECMA-262 and both of them say <code>this</code> is a keyword)...</p>
