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 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().
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.
AggregateError
Creates an instance representing several errors wrapped in a single error when multiple errors need to be reported by an operation, for example by Promise.any().
InternalError This API has not been standardized.
Creates an instance representing an error that occurs when an internal error in the JavaScript engine is thrown. E.g. "too much recursion".

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

Warning: Versions of Babel prior to 7 can handle CustomError class methods, but only when they are declared with Object.defineProperty(). Otherwise, old versions of Babel and other transpilers will not correctly handle the following code without additional configuration.

Note: Some browsers include the CustomError constructor in the stack trace when using ES2015 classes.

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

Warning: All browsers include the CustomError constructor in the stack trace when using a prototypal declaration.

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

Specification
ECMAScript Language Specification (ECMAScript)
#sec-error-objects

Browser compatibility

BCD tables only load in the browser

See also