spec.emu

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<pre class="metadata">
title: Error.isError
stage: 3
contributors: Jordan Harband
</pre>

<emu-clause id="sec-fundamental-objects" number="20">
	<h1>Fundamental Objects</h1>

	<emu-clause id="sec-error-objects" number="5">
		<h1>Error Objects</h1>

		<emu-clause id="sec-properties-of-the-error-constructor" number="2">
			<h1>Properties of the Error Constructor</h1>

			<emu-clause id="sec-error.iserror">
				<h1>Error.isError ( _arg_ )</h1>

				<emu-alg>
					1. Return IsError(_arg_).
				</emu-alg>
			</emu-clause>
		</emu-clause>

		<emu-clause id="sec-properties-of-error-instances">
            <h1>Properties of Error Instances</h1>

			<p>Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is *undefined*. The only specified uses of [[ErrorData]] is to identify Error, AggregateError, and _NativeError_ instances as Error objects within `Object.prototype.toString`<ins> and `Error.isError`</ins>.</p>
		</emu-clause>

		<emu-clause id="sec-properties-of-nativeerror-instances" number="6">
             <h1>Properties of _NativeError_ Instances</h1>
             <p>_NativeError_ instances are ordinary objects that inherit properties from their _NativeError_ prototype object and have an [[ErrorData]] internal slot whose value is *undefined*. The only specified use of [[ErrorData]] is by `Object.prototype.toString` (<emu-xref href="#sec-object.prototype.tostring"></emu-xref>) <ins> and `Error.isError` (<emu-xref href="#sec-error.iserror"></emu-xref>) </ins>to identify Error, AggregateError, or _NativeError_ instances.</p>
        </emu-clause>

		<emu-clause id="sec-aggregate-error-objects" number="7">
            <h1>AggregateError Objects</h1>

			<emu-clause id="sec-properties-of-aggregate-error-instances" number="3">
              <h1>Properties of AggregateError Instances</h1>
              <p>AggregateError instances are ordinary objects that inherit properties from their AggregateError prototype object and have an [[ErrorData]] internal slot whose value is *undefined*. The only specified use of [[ErrorData]] is by `Object.prototype.toString` (<emu-xref href="#sec-object.prototype.tostring"></emu-xref>) <ins> and `Error.isError` (<emu-xref href="#sec-error.iserror"></emu-xref>) </ins>to identify Error, AggregateError, or _NativeError_ instances.</p>
            </emu-clause>
		</emu-clause>

		<emu-clause id="sec-abstract-operations-for-error-objects" number="8">
			<h1>Abstract Operations for Error Objects</h1>

			<emu-clause id="sec-iserror" type="abstract operation" number="2" class="ins">
				<h1>
					IsError(
						_argument_: an Ecmascript language value,
					): a Boolean
				</h1>
				<dl class="header">
					<dt>description</dt>
					<dd>It returns a boolean indicating whether the argument is a built-in Error instance or not.</dd>
				</dl>

				<emu-alg>
					1. If _argument_ is not an Object, return *false*.
					1. If _argument_ has an [[ErrorData]] internal slot, return *true*.
					1. Return *false*.
				</emu-alg>
			</emu-clause>
		</emu-clause>
	</emu-clause>
</emu-clause>