Normative: add Error.isError

spec.html

@@ -31214,6 +31214,16 @@ <h1>Properties of the Error Constructor</h1>
         <li>has the following properties:</li>
       </ul>
 
+      <emu-clause id="sec-error.iserror">
+        <h1>Error.isError ( _arg_ )</h1>
+        <p>This function performs the following steps when called:</p>
+        <emu-alg>
+          1. If _arg_ is not an Object, return *false*.
+          1. If _arg_ does not have an [[ErrorData]] internal slot, return *false*.
+          1. Return *true*.
+        </emu-alg>
+      </emu-clause>
+
       <emu-clause id="sec-error.prototype">
         <h1>Error.prototype</h1>
         <p>The initial value of `Error.prototype` is the Error prototype object.</p>
@@ -31265,7 +31275,7 @@ <h1>Error.prototype.toString ( )</h1>
 
     <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`.</p>
+      <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` and `Error.isError`.</p>
     </emu-clause>
 
     <emu-clause id="sec-native-error-types-used-in-this-standard">
@@ -31381,7 +31391,7 @@ <h1>_NativeError_.prototype.name</h1>
 
       <emu-clause id="sec-properties-of-nativeerror-instances">
         <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>) to identify Error, AggregateError, or _NativeError_ instances.</p>
+        <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>) and `Error.isError` (<emu-xref href="#sec-error.iserror"></emu-xref>) to identify Error, AggregateError, or _NativeError_ instances.</p>
       </emu-clause>
     </emu-clause>
 
@@ -31458,7 +31468,7 @@ <h1>AggregateError.prototype.name</h1>
 
       <emu-clause id="sec-properties-of-aggregate-error-instances">
         <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>) to identify Error, AggregateError, or _NativeError_ instances.</p>
+        <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>) and `Error.isError` (<emu-xref href="#sec-error.iserror"></emu-xref>) to identify Error, AggregateError, or _NativeError_ instances.</p>
       </emu-clause>
     </emu-clause>