Revision 820927 of Error

<div>{{JSRef("Global_Objects", "Error", "EvalError,InternalError,RangeError,ReferenceError,SyntaxError,TypeError,URIError")}}</div>

<h2 id="Summary">Summary</h2>

<p>The <strong><code>Error</code></strong> constructor creates an error object. Instances of <code>Error</code> objects are thrown when runtime errors occur. The <code>Error</code> object can also be used as a base objects for user-defined exceptions. See below for standard built-in error types.</p>

<h2 id="Syntax" name="Syntax">Syntax</h2>

<pre class="syntaxbox">
new Error([<var>message</var>[, <var>fileName</var>[, <var>lineNumber</var>]]])</pre>

<h3 id="Parameters" name="Parameters">Parameters</h3>

<dl>
 <dt><code>message</code></dt>
 <dd>Optional. Human-readable description of the error.</dd>
 <dt><code>fileName</code> {{non-standard_inline}}</dt>
 <dd>Optional. The value for the <code>fileName</code> property on the created <code>Error</code> object. Defaults to the name of the file containing the code that called the <code>Error()</code> constructor.</dd>
 <dt><code>lineNumber</code> {{non-standard_inline}}</dt>
 <dd>Optional. The value for the <code>lineNumber</code> property on the created <code>Error</code> object. Defaults to the line number containing the <code>Error()</code> constructor invocation.</dd>
</dl>

<h2 id="Description" name="Description">Description</h2>

<p>Runtime errors result in new <code>Error</code> objects being created and thrown.</p>

<p>This page documents the use of the <code>Error</code> object itself and its use as a constructor function. For a list of properties and methods inherited by <code>Error</code> instances, see {{jsxref("Error.prototype")}}.</p>

<h3 id="Error_types" name="Error_types">Error types</h3>

<p>Besides the generic <code>Error</code> constructor, there are six other core error constructors in JavaScript. For client-side exceptions, see <a href="/en-US/docs/Web/JavaScript/Guide/Statements#Exception_Handling_Statements">Exception Handling Statements</a>.</p>

<dl>
 <dt>{{jsxref("Global_Objects/EvalError", "EvalError")}}</dt>
 <dd>Creates an instance representing an error that occurs regarding the global function {{jsxref("Global_Objects/eval", "eval()")}}.</dd>
 <dt>{{jsxref("Global_Objects/InternalError", "InternalError")}} {{non-standard_inline}}</dt>
 <dd>Creates an instance representing an error that occurs when an internal error in the JavaScript engine is thrown. E.g. "too much recursion".</dd>
 <dt>{{jsxref("Global_Objects/RangeError", "RangeError")}}</dt>
 <dd>Creates an instance representing an error that occurs when a numeric variable or parameter is outside of its valid range.</dd>
 <dt>{{jsxref("Global_Objects/ReferenceError", "ReferenceError")}}</dt>
 <dd>Creates an instance representing an error that occurs when de-referencing an invalid reference.</dd>
 <dt>{{jsxref("Global_Objects/SyntaxError", "SyntaxError")}}</dt>
 <dd>Creates an instance representing a syntax error that occurs while parsing code in {{jsxref("Global_Objects/eval", "eval()")}}.</dd>
 <dt>{{jsxref("Global_Objects/TypeError", "TypeError")}}</dt>
 <dd>Creates an instance representing an error that occurs when a variable or parameter is not of a valid type.</dd>
 <dt>{{jsxref("Global_Objects/URIError", "URIError")}}</dt>
 <dd>Creates an instance representing an error that occurs when {{jsxref("Global_Objects/encodeURI", "encodeURI()")}} or {{jsxref("Global_Objects/decodeURI", "decodeURI()")}} are passed invalid parameters.</dd>
</dl>

<h2 id="Properties" name="Properties">Properties</h2>

<dl>
 <dt>{{jsxref("Error.prototype")}}</dt>
 <dd>Allows the addition of properties to <code>Error</code> instances.</dd>
</dl>

<h2 id="Methods" name="Methods">Methods</h2>

<p>The global <code>Error</code> object contains no methods of its own, however, it does inherit some methods through the prototype chain.</p>

<h2 id="Error_instances" name="Error_instances"><code>Error</code> instances</h2>

<div>{{page('en-US/docs/JavaScript/Reference/Global_Objects/Error/prototype', 'Description')}}</div>

<h3 id="Properties_2">Properties</h3>

<div>{{page('en-US/docs/JavaScript/Reference/Global_Objects/Error/prototype', 'Properties')}}</div>

<h3 id="Methods_2">Methods</h3>

<div>{{page('en-US/docs/JavaScript/Reference/Global_Objects/Error/prototype', 'Methods')}}</div>

<h2 id="Examples" name="Examples">Examples</h2>

<h3 id="Example:_Throwing_a_generic_error" name="Example:_Throwing_a_generic_error">Example: Throwing a generic error</h3>

<p>Usually you create an <code>Error</code> object with the intention of raising it using the {{jsxref("Statements/throw", "throw")}} keyword. You can handle the error using the {{jsxref("Statements/try...catch", "try...catch")}} construct:</p>

<pre class="brush: js">
try {
  throw new Error('Whoops!');
} catch (e) {
  alert(e.name + ': ' + e.message);
}
</pre>

<h3 id="Example:_Handling_a_specific_error" name="Example:_Handling_a_specific_error">Example: Handling a specific error</h3>

<p><span class="comment">this should probably be removed</span> You can choose to handle only specific error types by testing the error type with the error's {{jsxref("Object.prototype.constructor", "constructor")}} property or, if you're writing for modern JavaScript engines, {{jsxref("Operators/instanceof", "instanceof")}} keyword:</p>

<pre class="brush: js">
try {
  foo.bar();
} catch (e) {
  if (e instanceof EvalError) {
    alert(e.name + ': ' + e.message);
  } else if (e instanceof RangeError) {
    alert(e.name + ': ' + e.message);
  }
  // ... etc
}
</pre>

<h3 id="Example.3A_Custom_Error_Types">Example: Custom Error Types</h3>

<p>You might want to define your own error types deriving from <code>Error</code> to be able to <code>throw new MyError()</code> and use <code>instanceof MyError</code> to check the kind of error in the exception handler. The common way to do this is demonstrated below.</p>

<div class="warning">
<p>Note that the thrown <code>MyError</code> will report incorrect <code>lineNumber</code> and <code>fileName</code> at least in Firefox.</p>
</div>

<p>See also the <a href="https://stackoverflow.com/questions/1382107/whats-a-good-way-to-extend-error-in-javascript">"What's a good way to extend Error in JavaScript?" discussion on Stackoverflow</a>.</p>

<pre class="brush: js">
// Create a new object, that prototypally inherits from the Error constructor.
function MyError(message) {
  this.name = 'MyError';
  this.message = message || 'Default Message';
}
MyError.prototype = Object.create(Error.prototype);
MyError.prototype.constructor = MyError;

try {
  throw new MyError();
} catch (e) {
  console.log(e.name);     // 'MyError'
  console.log(e.message);  // 'Default Message'
}

try {
  throw new MyError('custom message');
} catch (e) {
  console.log(e.name);     // 'MyError'
  console.log(e.message);  // 'custom message'
}
</pre>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>ECMAScript 1st Edition.</td>
   <td>Standard</td>
   <td>Initial definition. Implemented in JavaScript 1.1.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.11', 'Error')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-error-objects', 'Error')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<div>{{CompatibilityTable}}</div>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="See_also" name="See_also">See also</h2>

<ul>
 <li>{{jsxref("Error.prototype")}}</li>
 <li>{{jsxref("Statements/throw", "throw")}}</li>
 <li>{{jsxref("Statements/try...catch", "try...catch")}}</li>
</ul>