Errors#

Applications running in Node.js will generally experience the following categories of errors:

  • Standard JavaScript errors such as <EvalError>, <SyntaxError>, <RangeError>, <ReferenceError>, <TypeError>, and <URIError>.
  • Standard DOMExceptions.
  • System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist or attempting to send data over a closed socket.
  • AssertionErrors are a special class of error that can be triggered when Node.js detects an exceptional logic violation that should never occur. These are raised typically by the node:assert module.
  • User-specified errors triggered by application code.

All JavaScript and system errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.

The error.message property of errors raised by Node.js may be changed in any versions. Use error.code to identify an error instead. For a DOMException, use domException.name to identify its type.

Error propagation and interception#

Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called.

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try…catch construct provided by the JavaScript language.

// Throws with a ReferenceError because z is not defined.
try {
  const m = 1;
  const n = m + z;
} catch (err) {
  // Handle the error here.
}

js

Any use of the JavaScript throw mechanism will raise an exception that must be handled or the Node.js process will exit immediately.

With few exceptions, Synchronous APIs (any blocking method that does not return a <Promise> nor accept a callback function, such as fs.readFileSync), will use throw to report errors.

Errors that occur within Asynchronous APIs may be reported in multiple ways:

  • Some asynchronous methods returns a <Promise>, you should always take into account that it might be rejected. See --unhandled-rejections flag for how the process will react to an unhandled promise rejection.

    const fs = require('node:fs/promises');

(async () => {
  let data;
  try {
    data = await fs.readFile('a file that does not exist');
  } catch (err) {
    console.error('There was an error reading the file!', err);
    return;
  }
  // Otherwise handle the data
})();

    js

  • Most asynchronous methods that accept a callback function will accept an Error object passed as the first argument to that function. If that first argument is not null and is an instance of Error, then an error occurred that should be handled.

    const fs = require('node:fs');
fs.readFile('a file that does not exist', (err, data) => {
  if (err) {
    console.error('There was an error reading the file!', err);
    return;
  }
  // Otherwise handle the data
});

    js

  • When an asynchronous method is called on an object that is an EventEmitter, errors can be routed to that object's 'error' event.

    const net = require('node:net');
const connection = net.connect('localhost');

// Adding an 'error' event handler to a stream:
connection.on('error', (err) => {
  // If the connection is reset by the server, or if it can't
  // connect at all, or on any sort of error encountered by
  // the connection, the error will be sent here.
  console.error(err);
});

connection.pipe(process.stdout);

    js

  • A handful of typically asynchronous methods in the Node.js API may still use the throw mechanism to raise exceptions that must be handled using try…catch. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.

The use of the 'error' event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail).

For all EventEmitter objects, if an 'error' event handler is not provided, the error will be thrown, causing the Node.js process to report an uncaught exception and crash unless either: a handler has been registered for the 'uncaughtException' event, or the deprecated node:domain module is used.

const EventEmitter = require('node:events');
const ee = new EventEmitter();

setImmediate(() => {
  // This will crash the process because no 'error' event
  // handler has been added.
  ee.emit('error', new Error('This will crash'));
});

js

Errors generated in this way cannot be intercepted using try…catch as they are thrown after the calling code has already exited.

Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.

Class: Error#

A generic JavaScript <Error> object that does not denote any specific circumstance of why the error occurred. Error objects capture a "stack trace" detailing the point in the code at which the Error was instantiated, and may provide a text description of the error.

All errors generated by Node.js, including all system and JavaScript errors, will either be instances of, or inherit from, the Error class.

new Error(message[, options])#

Creates a new Error object and sets the error.message property to the provided text message. If an object is passed as message, the text message is generated by calling String(message). If the cause option is provided, it is assigned to the error.cause property. The error.stack property will represent the point in the code at which new Error() was called. Stack traces are dependent on V8's stack trace API. Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property Error.stackTraceLimit, whichever is smaller.

Error.captureStackTrace(targetObject[, constructorOpt])#

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called.

const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack;  // Similar to `new Error().stack`

js

The first line of the trace will be prefixed with ${myObject.name}: ${myObject.message}.

The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

The constructorOpt argument is useful for hiding implementation details of error generation from the user. For instance:

function a() {
  b();
}

function b() {
  c();
}

function c() {
  // Create an error without stack trace to avoid calculating the stack trace twice.
  const { stackTraceLimit } = Error;
  Error.stackTraceLimit = 0;
  const error = new Error();
  Error.stackTraceLimit = stackTraceLimit;

  // Capture the stack trace above function b
  Error.captureStackTrace(error, b); // Neither function c, nor b is included in the stack trace
  throw error;
}

a();

js

Error.stackTraceLimit#

The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

error.cause#

Added in: v16.9.0

If present, the error.cause property is the underlying cause of the Error. It is used when catching an error and throwing a new one with a different message or code in order to still have access to the original error.

The error.cause property is typically set by calling new Error(message, { cause }). It is not set by the constructor if the cause option is not provided.

This property allows errors to be chained. When serializing Error objects, util.inspect() recursively serializes error.cause if it is set.

const cause = new Error('The remote HTTP server responded with a 500 status');
const symptom = new Error('The message failed to send', { cause });

console.log(symptom);
// Prints:
//   Error: The message failed to send
//       at REPL2:1:17
//       at Script.runInThisContext (node:vm:130:12)
//       ... 7 lines matching cause stack trace ...
//       at [_line] [as _line] (node:internal/readline/interface:886:18) {
//     [cause]: Error: The remote HTTP server responded with a 500 status
//         at REPL1:1:15
//         at Script.runInThisContext (node:vm:130:12)
//         at REPLServer.defaultEval (node:repl:574:29)
//         at bound (node:domain:426:15)
//         at REPLServer.runBound [as eval] (node:domain:437:12)
//         at REPLServer.onLine (node:repl:902:10)
//         at REPLServer.emit (node:events:549:35)
//         at REPLServer.emit (node:domain:482:12)
//         at [_onLine] [as _onLine] (node:internal/readline/interface:425:12)
//         at [_line] [as _line] (node:internal/readline/interface:886:18)

js

error.code#

The error.code property is a string label that identifies the kind of error. error.code is the most stable way to identify an error. It will only change between major versions of Node.js. In contrast, error.message strings may change between any versions of Node.js. See Node.js error codes for details about specific codes.

error.message#

The error.message property is the string description of the error as set by calling new Error(message). The message passed to the constructor will also appear in the first line of the stack trace of the Error, however changing this property after the Error object is created may not change the first line of the stack trace (for example, when error.stack is read before this property is changed).

const err = new Error('The message');
console.error(err.message);
// Prints: The message

js

error.stack#

The error.stack property is a string describing the point in the code at which the Error was instantiated.

Error: Things keep happening!
   at /home/gbusey/file.js:525:2
   at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
   at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
   at increaseSynergy (/home/gbusey/actors.js:701:6)

console

The first line is formatted as <error class name>: <error message>, and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses.

Frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in the stack traces:

const cheetahify = require('./native-binding.node');

function makeFaster() {
  // `cheetahify()` *synchronously* calls speedy.
  cheetahify(function speedy() {
    throw new Error('oh no!');
  });
}

makeFaster();
// will throw:
//   /home/gbusey/file.js:6
//       throw new Error('oh no!');
//           ^
//   Error: oh no!
//       at speedy (/home/gbusey/file.js:6:11)
//       at makeFaster (/home/gbusey/file.js:5:3)
//       at Object.<anonymous> (/home/gbusey/file.js:10:1)
//       at Module._compile (module.js:456:26)
//       at Object.Module._extensions..js (module.js:474:10)
//       at Module.load (module.js:356:32)
//       at Function.Module._load (module.js:312:12)
//       at Function.Module.runMain (module.js:497:10)
//       at startup (node.js:119:16)
//       at node.js:906:3

js

The location information will be one of:

  • native, if the frame represents a call internal to V8 (as in [].forEach).
  • plain-filename.js:line:column, if the frame represents a call internal to Node.js.
  • /absolute/path/to/file.js:line:column, if the frame represents a call in a user program (using CommonJS module system), or its dependencies.
  • <transport-protocol>:///url/to/module/file.mjs:line:column, if the frame represents a call in a user program (using ES module system), or its dependencies.

The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick.

error.stack is a getter/setter for a hidden internal property which is only present on builtin Error objects (those for which Error.isError returns true). If error is not a builtin error object, then the error.stack getter will always return undefined, and the setter will do nothing. This can occur if the accessor is manually invoked with a this value that is not a builtin error object, such as a <Proxy>.

Class: AssertionError#

Indicates the failure of an assertion. For details, see Class: assert.AssertionError.

Class: DOMException#

Added in: v17.0.0

The Web IDL <DOMException> class. These errors are thrown by web-platform APIs in Node.js such as fetch(), <AbortController>, <AbortSignal>, and Web Streams. For details, see also Class: DOMException on the Globals page.

The domException.name property identifies the type of the exception (for example, 'AbortError'). Unlike most errors in Node.js, the domException.code property is a number that corresponds to a legacy error code name (for example, 20 for ABORT_ERR).

Node.js-specific APIs that support <AbortSignal> (such as events.once()) throw a Node.js AbortError (a native <errors.Error> with name of 'AbortError' and code of 'ABORT_ERR') rather than a <DOMException>. To identify abort errors in either case, checking err?.name === 'AbortError' is sufficient.

See also ABORT_ERR.

Class: RangeError#

Indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.

require('node:net').connect(-1);
// Throws "RangeError: "port" option should be >= 0 and < 65536: -1"

js

Node.js will generate and throw RangeError instances immediately as a form of argument validation.

Class: ReferenceError#

Indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.

While client code may generate and propagate these errors, in practice, only V8 will do so.

doesNotExist;
// Throws ReferenceError, doesNotExist is not a variable in this program.

js

Unless an application is dynamically generating and running code, ReferenceError instances indicate a bug in the code or its dependencies.

Class: SyntaxError#

Indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval, Function, require, or vm. These errors are almost always indicative of a broken program.

try {
  require('node:vm').runInThisContext('binary ! isNotOk');
} catch (err) {
  // 'err' will be a SyntaxError.
}

js

SyntaxError instances are unrecoverable in the context that created them – they may only be caught by other contexts.

Class: SystemError#

Node.js generates system errors when exceptions occur within its runtime environment. These usually occur when an application violates an operating system constraint. For example, a system error will occur if an application attempts to read a file that does not exist.

  • address <string> If present, the address to which a network connection failed
  • code <string> The string error code
  • dest <string> If present, the file path destination when reporting a file system error
  • errno <number> The system-provided error number
  • info <Object> If present, extra details about the error condition
  • message <string> A system-provided human-readable description of the error
  • path <string> If present, the file path when reporting a file system error
  • port <number> If present, the network connection port that is not available
  • syscall <string> The name of the system call that triggered the error

error.address#

If present, error.address is a string describing the address to which a network connection failed.

error.code#

The error.code property is a string representing the error code.

error.dest#

If present, error.dest is the file path destination when reporting a file system error.

error.errno#

The error.errno property is a negative number which corresponds to the error code defined in libuv Error handling.

On Windows the error number provided by the system will be normalized by libuv.

To get the string representation of the error code, use util.getSystemErrorName(error.errno).

error.info#

If present, error.info is an object with details about the error condition.

error.message#

error.message is a system-provided human-readable description of the error.

error.path#

If present, error.path is a string containing a relevant invalid pathname.

error.port#

If present, error.port is the network connection port that is not available.

error.syscall#

The error.syscall property is a string describing the syscall that failed.

Common system errors#

This is a list of system errors commonly-encountered when writing a Node.js program. For a comprehensive list, see the errno(3) man page.

  • EACCES (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.

  • EADDRINUSE (Address already in use): An attempt to bind a server (net, http, or https) to a local address failed due to another server on the local system already occupying that address.

  • ECONNREFUSED (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.

  • ECONNRESET (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the http and net modules.

  • EEXIST (File exists): An existing file was the target of an operation that required that the target not exist.

  • EISDIR (Is a directory): An operation expected a file, but the given pathname was a directory.

  • EMFILE (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, run ulimit -n 2048 in the same shell that will run the Node.js process.

  • ENOENT (No such file or directory): Commonly raised by fs operations to indicate that a component of the specified pathname does not exist. No entity (file or directory) could be found by the given path.

  • ENOTDIR (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir.

  • ENOTEMPTY (Directory not empty): A directory with entries was the target of an operation that requires an empty directory, usually fs.unlink.

  • ENOTFOUND (DNS lookup failed): Indicates a DNS failure of either EAI_NODATA or EAI_NONAME. This is not a standard POSIX error.

  • EPERM (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.

  • EPIPE (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the net and http layers, indicative that the remote side of the stream being written to has been closed.

  • ETIMEDOUT (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by http or net. Often a sign that a socket.end() was not properly called.

Class: TypeError#

Indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be a TypeError.

require('node:url').parse(() => { });
// Throws TypeError, since it expected a string.

js

Node.js will generate and throw TypeError instances immediately as a form of argument validation.

Exceptions vs. errors#

A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values are instances of Error or classes which inherit from Error, all exceptions thrown by Node.js or the JavaScript runtime will be instances of Error.

Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include assert() checks or abort() calls in the C++ layer.

OpenSSL errors#

Errors originating in crypto or tls are of class Error, and in addition to the standard .code and .message properties, may have some additional OpenSSL-specific properties.

error.opensslErrorStack#

An array of errors that can give context to where in the OpenSSL library an error originates from.

error.function#

The OpenSSL function the error originates in.

error.library#

The OpenSSL library the error originates in.

error.reason#

A human-readable string describing the reason for the error.

Node.js error codes#

ABORT_ERR#

Added in: v15.0.0

Used when an operation has been aborted (typically using an AbortController).

APIs not using AbortSignals typically do not raise an error with this code.

This code does not use the regular ERR_* convention Node.js errors use in order to be compatible with the web platform's AbortError.

ERR_ACCESS_DENIED#

A special type of error that is triggered whenever Node.js tries to get access to a resource restricted by the Permission Model.

ERR_AMBIGUOUS_ARGUMENT#

A function argument is being used in a way that suggests that the function signature may be misunderstood. This is thrown by the node:assert module when the message parameter in assert.throws(block, message) matches the error message thrown by block because that usage suggests that the user believes message is the expected message rather than the message the AssertionError will display if block does not throw.

ERR_ARG_NOT_ITERABLE#

An iterable argument (i.e. a value that works with for...of loops) was required, but not provided to a Node.js API.

ERR_ASSERTION#

A special type of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the node:assert module.

ERR_ASYNC_CALLBACK#

An attempt was made to register something that is not a function as an AsyncHooks callback.

ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED#

An operation related to module loading is customized by an asynchronous loader hook that never settled the promise before the loader thread exits.

ERR_ASYNC_TYPE#

The type of an asynchronous resource was invalid. Users are also able to define their own types if using the public embedder API.

ERR_BROTLI_COMPRESSION_FAILED#

Data passed to a Brotli stream was not successfully compressed.

ERR_BROTLI_INVALID_PARAM#

An invalid parameter key was passed during construction of a Brotli stream.

ERR_BUFFER_CONTEXT_NOT_AVAILABLE#

An attempt was made to create a Node.js Buffer instance from addon or embedder code, while in a JS engine Context that is not associated with a Node.js instance. The data passed to the Buffer method will have been released by the time the method returns.

When encountering this error, a possible alternative to creating a Buffer instance is to create a normal Uint8Array, which only differs in the prototype of the resulting object. Uint8Arrays are generally accepted in all Node.js core APIs where Buffers are; they are available in all Contexts.

ERR_BUFFER_OUT_OF_BOUNDS#

An operation outside the bounds of a Buffer was attempted.

ERR_BUFFER_TOO_LARGE#

An attempt has been made to create a Buffer larger than the maximum allowed size.

ERR_CANNOT_WATCH_SIGINT#

Node.js was unable to watch for the SIGINT signal.

ERR_CHILD_CLOSED_BEFORE_REPLY#

A child process was closed before the parent received a reply.

ERR_CHILD_PROCESS_IPC_REQUIRED#

Used when a child process is being forked without specifying an IPC channel.

ERR_CHILD_PROCESS_STDIO_MAXBUFFER#

Used when the main process is trying to read data from the child process's STDERR/STDOUT, and the data's length is longer than the maxBuffer option.

ERR_CLOSED_MESSAGE_PORT#

Added in: v10.5.0
History
VersionChanges
v16.2.0, v14.17.1

The error message was reintroduced.

v11.12.0

The error message was removed.

There was an attempt to use a MessagePort instance in a closed state, usually after .close() has been called.

ERR_CONSOLE_WRITABLE_STREAM