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QL code and tests for C#/C++/JavaScript.

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Pavel Avgustinov
2018-08-02 17:53:23 +01:00
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javascript/ql/src/Declarations/ArgumentsRedefined.qhelp Normal file
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<!DOCTYPE qhelp PUBLIC
"-//Semmle//qhelp//EN"
"qhelp.dtd">
<qhelp>
<overview>
<p>
JavaScript functions can access their arguments by position (rather than by parameter name)
through the special <code>arguments</code> object. However, if a function declares a parameter
or local variable named <code>arguments</code>, or assigns a new value to <code>arguments</code>,
then the <code>arguments</code> object is no longer available. This is confusing and makes code
hard to understand, so it should be avoided.
</p>
<p>
Also note that many popular JavaScript engines (such as V8, which is used by Google Chrome and
Node.js) do not support optimization of functions that assign to <code>arguments</code>, so such
functions will run more slowly.
</p>
</overview>
<recommendation>
<p>
Rename the variable to something else.
</p>
</recommendation>
<example>
<p>
In the following example, the <code>arguments</code> parameter of function <code>f</code> shadows the
special <code>arguments</code> variable. As a result, the <code>arguments</code> object cannot be
accessed inside <code>f</code>. To the casual reader, the test <code>x === arguments[0]</code> may look
redundant, since normally <code>arguments[0]</code> refers to the first argument (<code>x</code> in this
case), which would make the test trivially true. This is not the case here, however, since
<code>arguments[0]</code> refers to the first element of the array passed in as the second
argument.
</p>
<sample src="examples/ArgumentsRedefined.js" />
</example>
<references>
<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/arguments">arguments</a>.</li>
<li>Petka Antonov: <a href="https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments">Optimization killers</a>.</li>
</references>
</qhelp>