index.md

title	slug	tags	browser-compat
Error	Web/JavaScript/Reference/Global_Objects/Error	Class JavaScript Object Reference Polyfill	javascript.builtins.Error

{{JSRef}}

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 is a {{Glossary("serializable object")}}, so it can be cloned with {{domxref("structuredClone()")}} or copied between Workers using {{domxref("Worker.postMessage()", "postMessage()")}}.

Error types

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

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

Constructor

• {{jsxref("Error/Error", "Error()")}}
  • : Creates a new Error object.

Static methods

• Error.captureStackTrace() {{non-standard_inline}}

  • : A non-standard V8 function that creates the {{JSxRef("Error.prototype.stack", "stack")}} property on an Error instance.

• Error.stackTraceLimit {{non-standard_inline}}

  • : A non-standard V8 numerical property that limits how many stack frames to include in an error stacktrace.

• Error.prepareStackTrace() {{non-standard_inline}} {{optional_inline}}

  • : A non-standard V8 function that, if provided by usercode, is called by the V8 JavaScript engine for thrown exceptions, allowing the user to provide custom formatting for stacktraces.

Instance properties

• {{jsxref("Error.prototype.message")}}
  • : Error message. For user-created Error objects, this is the string provided as the constructor's first argument.
• {{jsxref("Error.prototype.name")}}
  • : Error name. This is determined by the constructor function.
• {{jsxref("Error.prototype.cause")}}
  • : Error cause indicating the reason why the current error is thrown — usually another caught error. For user-created Error objects, this is the value provided as the cause property of the constructor's second argument.
• {{jsxref("Error.prototype.fileName")}} {{non-standard_inline}}
  • : A non-standard Mozilla property for the path to the file that raised this error.
• {{jsxref("Error.prototype.lineNumber")}} {{non-standard_inline}}
  • : A non-standard Mozilla property for the line number in the file that raised this error.
• {{jsxref("Error.prototype.columnNumber")}} {{non-standard_inline}}
  • : A non-standard Mozilla property for the column number in the line that raised this error.
• {{jsxref("Error.prototype.stack")}} {{non-standard_inline}}
  • : A non-standard property for a stack trace.

Instance methods

• {{jsxref("Error.prototype.toString()")}}
  • : Returns a string representing the specified object. Overrides the {{jsxref("Object.prototype.toString()")}} method.

Examples

Throwing a generic error

Usually you create an Error 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:

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

Handling a specific error type

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:

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

  else {
    // If none of our cases matched leave the Error unhandled
    throw e;
  }
}

Differentiate between similar errors

Sometimes a block of code can fail for reasons that require different handling, but which throw very similar errors (i.e. with the same type and message).

If you don't have control over the original errors that are thrown, one option is to catch them and throw new Error objects that have more specific messages. The original error should be passed to the new Error in the constructor option parameter (cause property), as this ensures that the original error and stack trace are available to higher level try/catch blocks.

The example below shows this for two methods that would otherwise fail with similar errors (doFailSomeWay() and doFailAnotherWay()):

function doWork() {
  try {
    doFailSomeWay();
  } catch (err) {
    throw new Error('Failed in some way', { cause: err });
  }
  try {
    doFailAnotherWay();
  } catch (err) {
    throw new Error('Failed in another way', { cause: err });
  }
}

try {
  doWork();
} catch (err) {
  switch(err.message) {
    case 'Failed in some way':
      handleFailSomeWay(err.cause);
      break;
    case 'Failed in another way':
      handleFailAnotherWay(err.cause);
      break;
  }
}

Note: If you are making a library, you should prefer to use error cause to discriminate between different errors emitted — rather than asking your consumers to parse the error message. See the error cause page for an example.

Custom error types can also use the cause property, provided the subclasses' constructor passes the options parameter when calling super():

class MyError extends Error {
  constructor(/* some arguments */) {
    // Needs to pass both `message` and `options` to install the "cause" property.
    super(message, options);
  }
}

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 CustomError 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 CustomError 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.foo = foo;
  Object.setPrototypeOf(instance, CustomError.prototype);
  if (Error.captureStackTrace) {
    Error.captureStackTrace(instance, CustomError);
  }
  return instance;
}

Object.setPrototypeOf(CustomError.prototype, Error.prototype);

Object.setPrototypeOf(CustomError, Error);

CustomError.prototype.name = 'CustomError';

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

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• A polyfill of Error with modern behavior like support cause is available in core-js
• {{JSxRef("Statements/throw", "throw")}}
• {{JSxRef("Statements/try...catch", "try...catch")}}
• The V8 documentation for Error.captureStackTrace(), Error.stackTraceLimit, and Error.prepareStackTrace().