Error

Summary

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

Syntax

new Error([message[, fileName[, lineNumber]]])

Parameters

message

Optional. Human-readable description of the error.

fileName

Optional. The value for the fileName property on the created Error object. Defaults to the name of the file containing the code that called the Error() constructor.

lineNumber

Optional. The value for the lineNumber property on the created Error object. Defaults to the line number containing the Error() constructor invocation.

Description

Runtime errors result in new Error objects being created and thrown.

This page documents the use of the Error object itself and its use as a constructor function. For a list of properties and methods inherited by Error instances, see Error.prototype.

Error types

Besides the generic Error constructor, there are six other core error constructors in JavaScript. For client-side exceptions, see Exception Handling Statements.

EvalError

Creates an instance representing an error that occurs regarding the global function eval().

InternalError

Creates an instance representing an error that occurs when an internal error in the JavaScript engine is thrown. E.g. "too much recursion".

RangeError

Creates an instance representing an error that occurs when a numeric variable or parameter is outside of its valid range.

ReferenceError

Creates an instance representing an error that occurs when de-referencing an invalid reference.

SyntaxError

Creates an instance representing a syntax error that occurs while parsing code in eval().

TypeError

Creates an instance representing an error that occurs when a variable or parameter is not of a valid type.

URIError

Creates an instance representing an error that occurs when encodeURI() or decodeURl() are passed invalid parameters.

Properties

Error.prototype

Allows the addition of properties to Error instances.

Methods

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

Error instances

Properties

Methods

Examples

Example: Throwing a generic error

Usually you create an Error object with the intention of raising it using the throw keyword. You can handle the error using the try...catch construct:

try {
  throw new Error('Whoops!');
} catch (e) {
  alert(e.name + ': ' + e.message);
}

Example: Handling a specific error

this should probably be removed You can choose to handle only specific error types by testing the error type with the error's constructor property or, if you're writing for modern JavaScript engines, instanceof keyword:

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
}

Example: Custom Error Types

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

See also the "What's a good way to extend Error in JavaScript?" discussion on Stackoverflow.

// 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'
}

Specifications

Browser compatibility

See also

• Error.prototype
• throw
• try...catch