ShiftOutOfRange.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
Shift operations in JavaScript operate on 32-bit values only, so it is not possible
to shift by more than 31 positions. If the right operand of a shift operator is
greater than 31, the left operand is actually only shifted by that value modulo 32.
</p>

</overview>
<recommendation>

<p>
Use standard library functions such as <code>Math.pow</code> to perform the required
shifting. Alternatively, you can use the
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt">BigInt</a>
type if it is available on your platform.
</p>

</recommendation>
<example>

<p>
The following code snippet attempts to assign <code>x</code> the value 2<sup>40</sup>
(1099511627776). In fact, however, the left operand <code>1</code> is only shifted by
<code>8</code> (that is, 40 modulo 32), so <code>x</code> ends up being assigned
the value 2<sup>8</sup> (256).
</p>

<sample src="examples/ShiftOutOfRange.js" />

<p>
A better solution would be to use <code>Math.pow</code> as follows:
</p>

<sample src="examples/ShiftOutOfRangeGood.js" />

<p>
Note, however, that JavaScript internally represents large numbers as floating point
numbers, so numbers with a magnitude larger than 2<sup>53</sup> will be represented imprecisely.
</p>

</example>
<references>


<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators">Bitwise operators</a>.</li>


</references>
</qhelp>