Applications running in Node.js will generally experience four categories of errors:
eval() fails.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.
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 undefined
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 using try / catch or the Node.js process will exit immediately.
With few exceptions, Synchronous APIs (any blocking method that does not 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:
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('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('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 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 unhandled exception and crash unless either: The domain module is used appropriately or a handler has been registered for the process.on('uncaughtException') event.
const EventEmitter = require('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.
Most asynchronous methods exposed by the Node.js core API follow an idiomatic pattern referred to as an error-first callback (sometimes referred to as a Node.js style callback). With this pattern, a callback function is passed to the method as an argument. When the operation either completes or an error is raised, the callback function is called with the Error object (if any) passed as the first argument. If no error was raised, the first argument will be passed as null.
const fs = require('fs');
function errorFirstCallback(err, data) {
if (err) {
console.error('There was an error', err);
return;
}
console.log(data);
}
fs.readFile('/some/file/that/does-not-exist', errorFirstCallback);
fs.readFile('/some/file/that/does-exist', errorFirstCallback);
The JavaScript try / catch mechanism cannot be used to intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside an error-first callback:
// THIS WILL NOT WORK:
const fs = require('fs');
try {
fs.readFile('/some/file/that/does-not-exist', (err, data) => {
// mistaken assumption: throwing here...
if (err) {
throw err;
}
});
} catch (err) {
// This will not catch the throw!
console.error(err);
}
This will not work because the callback function passed to fs.readFile() is called asynchronously. By the time the callback has been called, the surrounding code (including the try { } catch (err) { } block will have already exited. Throwing an error inside the callback can crash the Node.js process in most cases. If domains are enabled, or a handler has been registered with process.on('uncaughtException'), such errors can be intercepted.
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.
For crypto only, Error objects will include the OpenSSL error stack in a separate property called opensslErrorStack if it is available when the error is thrown.
All errors generated by Node.js, including all System and JavaScript errors, will either be instances of, or inherit from, the Error class.
message <string>
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 message.toString(). 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.
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 an end user. For instance:
function MyError() {
Error.captureStackTrace(this, MyError);
}
// Without passing MyError to captureStackTrace, the MyError
// frame would show up in the .stack property. By passing
// the constructor, we omit that frame, and retain all frames below it.
new MyError().stack;
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.
The error.code property is a string label that identifies the kind of error. See Node.js Error Codes for details about specific codes.
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
The error.stack property is a string describing the point in the code at which the Error was instantiated.
For example:
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, 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.
System-level errors are generated as augmented Error instances, which are detailed here.
A subclass of Error that indicates the failure of an assertion. Such errors commonly indicate inequality of actual and expected value.
For example:
assert.strictEqual(1, 2); // AssertionError [ERR_ASSERTION]: 1 === 2
A subclass of Error that 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.
For example:
require('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.
A subclass of Error that 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 should always be considered a bug in the code or its dependencies.
A subclass of Error that 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('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.
A subclass of Error that indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError.
require('url').parse(() => { });
// throws TypeError, since it expected a string
Node.js will generate and throw TypeError instances immediately as a form of argument validation.
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.
System errors are generated when exceptions occur within the program's runtime environment. Typically, these are operational errors that occur when an application violates an operating system constraint such as attempting to read a file that does not exist or when the user does not have sufficient permissions.
System errors are typically generated at the syscall level: an exhaustive list of error codes and their meanings is available by running man 2 intro or man 3 errno on most Unices; or online.
In Node.js, system errors are represented as augmented Error objects with added properties.
The error.code property is a string representing the error code, which is typically E followed by a sequence of capital letters.
The error.errno property is a number or a string. The number is a negative value which corresponds to the error code defined in libuv Error handling. See uv-errno.h header file (deps/uv/include/uv-errno.h in the Node.js source tree) for details. In case of a string, it is the same as error.code.
The error.syscall property is a string describing the syscall that failed.
When present (e.g. in fs or child_process), the error.path property is a string containing a relevant invalid pathname.
When present (e.g. in net or dgram), the error.address property is a string describing the address to which the connection failed.
When present (e.g. in net or dgram), the error.port property is a number representing the connection's port that is not available.
This list is not exhaustive, but enumerates many of the common system errors encountered when writing a Node.js program. An exhaustive list may be found here.
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.
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.
An iterable argument (i.e. a value that works with for...of loops) was required, but not provided to a Node.js API.
An attempt was made to register something that is not a function as an AsyncHooks callback.
The type of an asynchronous resource was invalid. Note that users are also able to define their own types if using the public embedder API.
Data provided to util.TextDecoder() API was invalid according to the encoding provided.
Encoding provided to util.TextDecoder() API was not one of the WHATWG Supported Encodings.
A Promise that was callbackified via util.callbackify() was rejected with a falsy value.
An attempt was made to add more headers after the headers had already been sent.
An invalid character was found in an HTTP response status message (reason phrase).
Status code was outside the regular status code range (100-999).
The Trailer header was set even though the transfer encoding does not support that.
Occurs with multiple attempts to shutdown an HTTP/2 session.
HTTP/2 ALTSVC frames require a valid origin.
HTTP/2 ALTSVC frames are limited to a maximum of 16,382 payload bytes.
For HTTP/2 requests using the CONNECT method, the :authority pseudo-header is required.
For HTTP/2 requests using the CONNECT method, the :path pseudo-header is forbidden.
For HTTP/2 requests using the CONNECT method, the :scheme pseudo-header is forbidden.
A failure occurred sending an individual frame on the HTTP/2 session.
New HTTP/2 Streams may not be opened after the Http2Session has received a GOAWAY frame from the connected peer.
A required header was missing in an HTTP/2 message.
Multiple values were provided for an HTTP/2 header field that was required to have only a single value.
An additional headers was specified after an HTTP/2 response was initiated.
An HTTP/2 Headers Object was expected.
An attempt was made to send multiple response headers.
HTTP/2 Informational headers must only be sent prior to calling the Http2Stream.prototype.respond() method.
Informational HTTP status codes (1xx) may not be set as the response status code on HTTP/2 responses.
HTTP/1 connection specific headers are forbidden to be used in HTTP/2 requests and responses.
An invalid HTTP/2 header value was specified.
An invalid HTTP informational status code has been specified. Informational status codes must be an integer between 100 and 199 (inclusive).
Input Buffer and Uint8Array instances passed to the http2.getUnpackedSettings() API must have a length that is a multiple of six.
Only valid HTTP/2 pseudoheaders (:status, :path, :authority, :scheme, and :method) may be used.
An action was performed on an Http2Session object that had already been destroyed.
An invalid value has been specified for an HTTP/2 setting.
An operation was performed on a stream that had already been destroyed.
Whenever an HTTP/2 SETTINGS frame is sent to a connected peer, the peer is required to send an acknowledgment that it has received and applied the new SETTINGS. By default, a maximum number of unacknowledged SETTINGS frames may be sent at any given time. This error code is used when that limit has been reached.
An attempt was made to directly manipulate (read, write, pause, resume, etc.) a socket attached to an Http2Session.
The number of streams created on a single HTTP/2 session reached the maximum limit.
A message payload was specified for an HTTP response code for which a payload is forbidden.
An HTTP/2 ping was canceled.
HTTP/2 ping payloads must be exactly 8 bytes in length.
An HTTP/2 pseudo-header has been used inappropriately. Pseudo-headers are header key names that begin with the : prefix.
An attempt was made to create a push stream, which had been disabled by the client.
An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send something other than a regular file.
The Http2Session closed with a non-zero error code.
An attempt was made to connect a Http2Session object to a net.Socket or tls.TLSSocket that had already been bound to another Http2Session object.
Use of the 101 Informational status code is forbidden in HTTP/2.
An invalid HTTP status code has been specified. Status codes must be an integer between 100 and 599 (inclusive).
An Http2Stream was destroyed before any data was transmitted to the connected peer.
A non-zero error code was been specified in an RST_STREAM frame.
When setting the priority for an HTTP/2 stream, the stream may be marked as a dependency for a parent stream. This error code is used when an attempt is made to mark a stream and dependent of itself.
http2.connect() was passed a URL that uses any protocol other than http: or https:.
A given index was out of the accepted range (e.g. negative offsets).
An argument of the wrong type was passed to a Node.js API.
An invalid asyncId or triggerAsyncId was passed using AsyncHooks. An id less than -1 should never happen.
A callback function was required but was not been provided to a Node.js API.
A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible host. This situation can only occur on Unix-like systems where only localhost or an empty host is supported.
A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible path. The exact semantics for determining whether a path can be used is platform-dependent.
An attempt was made to send an unsupported "handle" over an IPC communication channel to a child process. See subprocess.send() and process.send() for more information.
An invalid or unexpected value was passed in an options object.
While using the Performance Timing API (perf_hooks), a performance mark is invalid.
An invalid options.protocol was passed.
A Buffer, Uint8Array or string was provided as stdio input to a synchronous fork. See the documentation for the child_process module for more information.
A Node.js API function was called with an incompatible this value.
Example:
const { URLSearchParams } = require('url');
const urlSearchParams = new URLSearchParams('foo=bar&baz=new');
const buf = Buffer.alloc(1);
urlSearchParams.has.call(buf, 'foo');
// Throws a TypeError with code 'ERR_INVALID_THIS'
An element in the iterable provided to the WHATWG URLSearchParams constructor did not represent a [name, value] tuple – that is, if an element is not iterable, or does not consist of exactly two elements.
An invalid URL was passed to the WHATWG URL constructor to be parsed. The thrown error object typically has an additional property 'input' that contains the URL that failed to parse.
An attempt was made to use a URL of an incompatible scheme (protocol) for a specific purpose. It is only used in the WHATWG URL API support in the fs module (which only accepts URLs with 'file' scheme), but may be used in other Node.js APIs as well in the future.
An attempt was made to use an IPC communication channel that was already closed.
An attempt was made to disconnect an IPC communication channel that was already disconnected. See the documentation for the child_process module for more information.
An attempt was made to create a child Node.js process using more than one IPC communication channel. See the documentation for the child_process module for more information.
An attempt was made to open an IPC communication channel with a synchronously forked Node.js process. See the documentation for the child_process module for more information.
A required argument of a Node.js API was not passed. This is only used for strict compliance with the API specification (which in some cases may accept func(undefined) but not func()). In most native Node.js APIs, func(undefined) and func() are treated identically, and the ERR_INVALID_ARG_TYPE error code may be used instead.
Used when an [ES6 module][] loader hook specifies format: 'dynamic but does not provide a dynamicInstantiate hook.
Used when an [ES6 module][] cannot be resolved.
Used when a failure occurred resolving imports in an [ES6 module][].
A callback was called more than once.
Note: A callback is almost always meant to only be called once as the query can either be fulfilled or rejected but not both at the same time. The latter would be possible by calling a callback more than once.
While using N-API, a constructor passed was not a function.
While using N-API, Constructor.prototype was not an object.
While calling napi_create_dataview(), a given offset was outside the bounds of the dataview or offset + length was larger than a length of given buffer.
While calling napi_create_typedarray(), the provided offset was not a multiple of the element size.
While calling napi_create_typedarray(), (length * size_of_element) +
byte_offset was larger than the length of given buffer.
An attempt was made to use features that require ICU, but Node.js was not compiled with ICU support.
An attempt was made to bind a socket that has already been bound.
An API function expecting a port > 0 and < 65536 received an invalid value.
An API function expecting a socket type (udp4 or udp6) received an invalid value.
Data could be sent on a socket.
An attempt was made to operate on an already closed socket.
A call was made and the UDP subsystem was not running.
An attempt was made to close the process.stderr stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code.
An attempt was made to close the process.stdout stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code.
While using TLS, the hostname/IP of the peer did not match any of the subjectAltNames in its certificate.
While using TLS, the parameter offered for the Diffie-Hellman (DH) key-agreement protocol is too small. By default, the key length must be greater than or equal to 1024 bits to avoid vulnerabilities, even though it is strongly recommended to use 2048 bits or larger for stronger security.
A TLS/SSL handshake timed out. In this case, the server must also abort the connection.
A TLS renegotiation request has failed in a non-specific way.
While using TLS, the server.addContext() method was called without providing a hostname in the first parameter.
An excessive amount of TLS renegotiations is detected, which is a potential vector for denial-of-service attacks.
A Transform stream finished while it was still transforming.
A Transform stream finished with data still in the write buffer.
An invalid or unknown process signal was passed to an API expecting a valid signal (such as subprocess.kill()).
An attempt was made to launch a Node.js process with an unknown stdin file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.
An attempt was made to launch a Node.js process with an unknown stdout or stderr file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.
The V8 BreakIterator API was used but the full ICU data set is not installed.
While using the Performance Timing API (perf_hooks), no valid performance entry types were found.
A given value is out of the accepted range.
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https://nodejs.org/dist/latest-v8.x/docs/api/errors.html