<!DOCTYPE qhelp PUBLIC

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<qhelp>

<overview>

<p>

In ECMAScript 5, the <code>typeof</code> operator returns one of the following six type tags:

<code>"undefined"</code>, <code>"boolean"</code>, <code>"number"</code>, <code>"string"</code>,

<code>"object"</code>, <code>"function"</code>. In ECMAScript 2015, it may additionally return

<code>"symbol"</code>, while on older versions of Internet Explorer it may return

<code>"unknown"</code> or <code>"date"</code> in certain situations. Comparing it against

any other string literal is therefore useless.

</p>

</overview>

<recommendation>

<p>

Carefully examine the comparison in question. If the type tag is simply misspelled, correct it.

In some cases, the type tag returned by <code>typeof</code> is not sufficiently precise, so

you may have to use other type test functions.

</p>

</recommendation>

<example>

<p>

The following code snippet tries to determine whether <code>a</code> is an array:

</p>

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

<p>

Note that <code>typeof</code> is not precise enough to distinguish arrays from other objects,

since it returns the type tag <code>"object"</code> for both. ECMAScript 5-compatible platforms

provide a library function <code>Array.isArray</code> that can be used instead:

</p>

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

<p>

On older platforms, you can use the technique explained on the

<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray#Compatibility">Mozilla Developer Network</a>.

</p>

</example>

<references>

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

<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray">Array.isArray()</a>.</li>

</references>

</qhelp>