Errors
Applications running in Node.js will generally experience four categories of errors:
- Standard JavaScript errors such as {EvalError}, {SyntaxError}, {RangeError}, {ReferenceError}, {TypeError}, and {URIError}.
- 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.
- User-specified errors triggered by application code.
AssertionError
s 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 thenode:assert
module.
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.
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.
}
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('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
})(); -
Most asynchronous methods that accept a
callback
function will accept anError
object passed as the first argument to that function. If that first argument is notnull
and is an instance ofError
, 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
}); -
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); -
A handful of typically asynchronous methods in the Node.js API may still use the
throw
mechanism to raise exceptions that must be handled usingtry…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'));
});
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])
message
{string}options
{Object}cause
{any} The error that caused the newly created error.
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])
targetObject
{Object}constructorOpt
{Function}
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`
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();
Error.stackTraceLimit
- {number}
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
- {any}
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)
error.code
- {string}
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
- {string}
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
error.stack
- {string}
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)
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
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 string representing the stack trace is lazily generated when the
error.stack
property is accessed.
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.
Class: AssertionError
- Extends: {errors.Error}
Indicates the failure of an assertion. For details, see
Class: assert.AssertionError
.
Class: RangeError
- Extends: {errors.Error}
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"
Node.js will generate and throw RangeError
instances immediately as a form
of argument validation.
Class: ReferenceError
- Extends: {errors.Error}
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.
Unless an application is dynamically generating and running code,
ReferenceError
instances indicate a bug in the code or its dependencies.
Class: SyntaxError
- Extends: {errors.Error}
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.
}
SyntaxError
instances are unrecoverable in the context that created them –
they may only be caught by other contexts.
Class: SystemError
- Extends: {errors.Error}
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 failedcode
{string} The string error codedest
{string} If present, the file path destination when reporting a file system errorerrno
{number} The system-provided error numberinfo
{Object} If present, extra details about the error conditionmessage
{string} A system-provided human-readable description of the errorpath
{string} If present, the file path when reporting a file system errorport
{number} If present, the network connection port that is not availablesyscall
{string} The name of the system call that triggered the error
error.address
- {string}
If present, error.address
is a string describing the address to which a
network connection failed.
error.code
- {string}
The error.code
property is a string representing the error code.
error.dest
- {string}
If present, error.dest
is the file path destination when reporting a file
system error.
error.errno
- {number}
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
- {Object}
If present, error.info
is an object with details about the error condition.
error.message
- {string}
error.message
is a system-provided human-readable description of the error.
error.path
- {string}
If present, error.path
is a string containing a relevant invalid pathname.
error.port
- {number}
If present, error.port
is the network connection port that is not available.
error.syscall
- {string}
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
, orhttps
) 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 thehttp
andnet
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, runulimit -n 2048
in the same shell that will run the Node.js process. -
ENOENT
(No such file or directory): Commonly raised byfs
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 byfs.readdir
. -
ENOTEMPTY
(Directory not empty): A directory with entries was the target of an operation that requires an empty directory, usuallyfs.unlink
. -
ENOTFOUND
(DNS lookup failed): Indicates a DNS failure of eitherEAI_NODATA
orEAI_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 thenet
andhttp
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 byhttp
ornet
. Often a sign that asocket.end()
was not properly called.
Class: TypeError
- Extends {errors.Error}
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.
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
Used when an operation has been aborted (typically using an AbortController
).
APIs not using AbortSignal
s 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_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. Uint8Array
s are generally accepted in all
Node.js core APIs where Buffer
s 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
There was an attempt to use a MessagePort
instance in a closed
state, usually after .close()
has been called.
ERR_CONSOLE_WRITABLE_STREAM
Console
was instantiated without stdout
stream, or Console
has a
non-writable stdout
or stderr
stream.
ERR_CONSTRUCT_CALL_INVALID
A class constructor was called that is not callable.
ERR_CONSTRUCT_CALL_REQUIRED
A constructor for a class was called without new
.
ERR_CONTEXT_NOT_INITIALIZED
The vm context passed into the API is not yet initialized. This could happen when an error occurs (and is caught) during the creation of the context, for example, when the allocation fails or the maximum call stack size is reached when the context is created.
ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED
An OpenSSL engine was requested (for example, through the clientCertEngine
or
privateKeyEngine
TLS options) that is not supported by the version of OpenSSL
being used, likely due to the compile-time flag OPENSSL_NO_ENGINE
.
ERR_CRYPTO_ECDH_INVALID_FORMAT
An invalid value for the format
argument was passed to the crypto.ECDH()
class getPublicKey()
method.
ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY
An invalid value for the key
argument has been passed to the
crypto.ECDH()
class computeSecret()
method. It means that the public
key lies outside of the elliptic curve.
ERR_CRYPTO_ENGINE_UNKNOWN
An invalid crypto engine identifier was passed to
require('node:crypto').setEngine()
.
ERR_CRYPTO_FIPS_FORCED
The --force-fips
command-line argument was used but there was an attempt
to enable or disable FIPS mode in the node:crypto
module.
ERR_CRYPTO_FIPS_UNAVAILABLE
An attempt was made to enable or disable FIPS mode, but FIPS mode was not available.
ERR_CRYPTO_HASH_FINALIZED
hash.digest()
was called multiple times. The hash.digest()
method must
be called no more than one time per instance of a Hash
object.
ERR_CRYPTO_HASH_UPDATE_FAILED
hash.update()
failed for any reason. This should rarely, if ever, happen.
ERR_CRYPTO_INCOMPATIBLE_KEY
The given crypto keys are incompatible with the attempted operation.
ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS
The selected public or private key encoding is incompatible with other options.
ERR_CRYPTO_INITIALIZATION_FAILED
Initialization of the crypto subsystem failed.
ERR_CRYPTO_INVALID_AUTH_TAG
An invalid authentication tag was provided.
ERR_CRYPTO_INVALID_COUNTER
An invalid counter was provided for a counter-mode cipher.
ERR_CRYPTO_INVALID_CURVE
An invalid elliptic-curve was provided.
ERR_CRYPTO_INVALID_DIGEST
An invalid crypto digest algorithm was specified.
ERR_CRYPTO_INVALID_IV
An invalid initialization vector was provided.
ERR_CRYPTO_INVALID_JWK
An invalid JSON Web Key was provided.
ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE
The given crypto key object's type is invalid for the attempted operation.
ERR_CRYPTO_INVALID_KEYLEN
An invalid key length was provided.
ERR_CRYPTO_INVALID_KEYPAIR
An invalid key pair was provided.
ERR_CRYPTO_INVALID_KEYTYPE
An invalid key type was provided.
ERR_CRYPTO_INVALID_MESSAGELEN
An invalid message length was provided.
ERR_CRYPTO_INVALID_SCRYPT_PARAMS
One or more crypto.scrypt()
or crypto.scryptSync()
parameters are
outside their legal range.
ERR_CRYPTO_INVALID_STATE
A crypto method was used on an object that was in an invalid state. For
instance, calling cipher.getAuthTag()
before calling cipher.final()
.
ERR_CRYPTO_INVALID_TAG_LENGTH
An invalid authentication tag length was provided.
ERR_CRYPTO_JOB_INIT_FAILED
Initialization of an asynchronous crypto operation failed.
ERR_CRYPTO_JWK_UNSUPPORTED_CURVE
Key's Elliptic Curve is not registered for use in the JSON Web Key Elliptic Curve Registry.
ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE
Key's Asymmetric Key Type is not registered for use in the JSON Web Key Types Registry.
ERR_CRYPTO_OPERATION_FAILED
A crypto operation failed for an otherwise unspecified reason.
ERR_CRYPTO_PBKDF2_ERROR
The PBKDF2 algorithm failed for unspecified reasons. OpenSSL does not provide more details and therefore neither does Node.js.
ERR_CRYPTO_SCRYPT_NOT_SUPPORTED
Node.js was compiled without scrypt
support. Not possible with the official
release binaries but can happen with custom builds, including distro builds.
ERR_CRYPTO_SIGN_KEY_REQUIRED
A signing key
was not provided to the sign.sign()
method.
ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH
crypto.timingSafeEqual()
was called with Buffer
, TypedArray
, or
DataView
arguments of different lengths.
ERR_CRYPTO_UNKNOWN_CIPHER
An unknown cipher was specified.
ERR_CRYPTO_UNKNOWN_DH_GROUP
An unknown Diffie-Hellman group name was given. See
crypto.getDiffieHellman()
for a list of valid group names.