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
, so it can be cloned with
or copied between Workers using
.
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 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.
-
- : 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.
- : Creates a new
Static methods
Error.captureStackTrace()
- : A non-standard V8 function that creates the stack property on an Error instance.
Error.stackTraceLimit
- : A non-standard V8 numerical property that limits how many stack frames to include in an error stacktrace.
Error.prepareStackTrace()
- : 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
These properties are defined on Error.prototype
and shared by all Error
instances.
- Error.prototype.constructor
- : The constructor function that created the instance object. For
Error
instances, the initial value is the Error constructor.
- : The constructor function that created the instance object. For
- Error.prototype.name
- : Represents the name for the type of error. For
Error.prototype.name
, the initial value is"Error"
. Subclasses like TypeError and SyntaxError provide their ownname
properties.
- : Represents the name for the type of error. For
-
- : A non-standard property for a stack trace.
These properties are own properties of each Error
instance.
- 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 thecause
property of the constructor's second argument.
- : Error cause indicating the reason why the current error is thrown — usually another caught error. For user-created
-
- : A non-standard Mozilla property for the column number in the line that raised this error.
-
- : A non-standard Mozilla property for the path to the file that raised this error.
-
- : A non-standard Mozilla property for the line number in the file that raised this error.
- message
- : Error message. For user-created
Error
objects, this is the string provided as the constructor's first argument.
- : Error message. For user-created
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}: ${e.message}`);
}
Handling a specific error type
You can choose to handle only specific error types by testing the error type with the instanceof keyword:
try {
foo.bar();
} catch (e) {
if (e instanceof EvalError) {
console.error(`${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's options
parameter as its cause
property. 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()
. The Error()
base class constructor will read options.cause
and define the cause
property on the new error instance.
class MyError extends Error {
constructor(message, options) {
// Need to pass `options` as the second parameter to install the "cause" property.
super(message, options);
}
}
console.log(new MyError("test", { cause: new Error("cause") }).cause);
// Error: cause
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.
Warning: Builtin subclassing cannot be reliably transpiled to pre-ES6 code, because there's no way to construct the base class with a particular
new.target
without Reflect.construct. You need additional configuration or manually call Object.setPrototypeOf(this, CustomError.prototype) at the end of the constructor; otherwise, the constructed instance will not be aCustomError
instance. See the TypeScript FAQ for more information.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
}