Error

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

Description

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

Error types

Besides the generic Error constructor, there are seven 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.

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 decodeURI() are passed invalid parameters.

Constructor

Error()

Creates a new Error object.

Static methods

Error.captureStackTrace()

A non-standard V8 function that creates the stack property on an Error instance.

Instance properties

Error.prototype.message

Error message.

Error.prototype.name

Error name.

Error.prototype.description

A non-standard Microsoft property for the error description. Similar to message.

Error.prototype.number

A non-standard Microsoft property for an error number.

Error.prototype.fileName

A non-standard Mozilla property for the path to the file that raised this error.

Error.prototype.lineNumber

A non-standard Mozilla property for the line number in the file that raised this error.

Error.prototype.columnNumber

A non-standard Mozilla property for the column number in the line that raised this error.

Error.prototype.stack

A non-standard Mozilla property for a stack trace.

Instance methods

Error.prototype.toString()

Returns a string representing the specified object. Overrides the Object.prototype.toString() method.

Examples

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) {
  console.error(e.name + ': ' + e.message)
}

Handling a specific error

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) {
    console.error(e.name + ': ' + e.message)
  } else if (e instanceof RangeError) {
    console.error(e.name + ': ' + e.message)
  }
  // ... etc
}

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. This results in cleaner and more consistent error handling code.

See "What's a good way to extend Error in JavaScript?" on StackOverflow for an in-depth discussion.

ES6 Custom Error Class

class CustomError extends Error {
  constructor(foo = 'bar', ...params) {
    // Pass remaining arguments (including vendor specific ones) to parent constructor
    super(...params)

    // Maintains proper stack trace for where our error was thrown (only available on V8)
    if (Error.captureStackTrace) {
      Error.captureStackTrace(this, CustomError)
    }

    this.name = 'CustomError'
    // Custom debugging information
    this.foo = foo
    this.date = new Date()
  }
}

try {
  throw new CustomError('baz', 'bazMessage')
} catch(e) {
  console.error(e.name)    //CustomError
  console.error(e.foo)     //baz
  console.error(e.message) //bazMessage
  console.error(e.stack)   //stacktrace
}

ES5 Custom Error Object

function CustomError(foo, message, fileName, lineNumber) {
  var instance = new Error(message, fileName, lineNumber);
  instance.name = 'CustomError';
  instance.foo = foo;
  Object.setPrototypeOf(instance, Object.getPrototypeOf(this));
  if (Error.captureStackTrace) {
    Error.captureStackTrace(instance, CustomError);
  }
  return instance;
}

CustomError.prototype = Object.create(Error.prototype, {
  constructor: {
    value: Error,
    enumerable: false,
    writable: true,
    configurable: true
  }
});

if (Object.setPrototypeOf){
  Object.setPrototypeOf(CustomError, Error);
} else {
  CustomError.__proto__ = Error;
}

try {
  throw new CustomError('baz', 'bazMessage');
} catch(e){
  console.error(e.name); //CustomError
  console.error(e.foo); //baz
  console.error(e.message); //bazMessage
}

Specifications

Browser compatibility

See also

• throw
• try...catch