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Errors

Errors generated by Node.js fall into two categories: JavaScript errors and system errors. All errors inherit from or are instances of JavaScript's Error class and are guaranteed to provide at least the attributes available on that class.

When an operation is not permitted due to language-syntax or language-runtime-level reasons, a JavaScript error is generated and thrown as an exception. If an operation is not allowed due to system-level restrictions, a system error is generated. Client code is then given the opportunity to intercept this error based on how the API propagates it.

The style of API called determines how generated errors are handed back, or propagated, to client code, which in turn informs how the client may intercept the error. Exceptions can be intercepted using the try / catch construct; other propagation strategies are covered below.

Error Propagation and Interception

All Node.js APIs will treat invalid arguments as exceptional -- that is, if passed invalid arguments, they will immediately generate and throw the error as an exception, even if they are an otherwise asynchronous API.

Synchronous APIs (like fs.readFileSync) will throw the error. The act of throwing a value (in this case, the error) turns the value into an exception. Exceptions may be caught using the try { } catch(err) { } construct.

Asynchronous APIs have two mechanisms for error propagation; one mechanism for APIs that represent a single operation, and one for APIs that represent multiple operations over time.

Error events

The other mechanism for providing errors is the 'error' event. This is typically used by stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (versus a single operation that may pass or fail). If no 'error' event handler is attached to the source of the error, the error will be thrown. At this point, it will crash the process as an unhandled exception unless domains are employed appropriately or process.on('uncaughtException') has a handler.

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);

The "throw when no error handlers are attached behavior" is not limited to APIs provided by Node.js -- even user created event emitters and streams will throw errors when no error handlers are attached. An example:

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'));
});

As with node style callbacks, errors generated this way cannot be intercepted by try { } catch(err) { } -- they happen after the calling code has already exited.

Node style callbacks

Single operation APIs take "node style callbacks" -- a function provided to the API as an argument. The node style callback takes at least one argument -- error -- that will either be null (if no error was encountered) or an Error instance. For instance:

const fs = require('fs');

fs.readFile('/some/file/that/does-not-exist', function nodeStyleCallback(err, data) {
  console.log(err)  // Error: ENOENT
  console.log(data) // undefined / null
});

fs.readFile('/some/file/that/does-exist', (err, data) => {
  console.log(err)  // null
  console.log(data) // <Buffer: ba dd ca fe>
})

Note that try { } catch(err) { } cannot intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside their node style 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) {
  // ... will be caught here -- this is incorrect!
  console.log(err); // Error: ENOENT
}

This will not work! By the time the node style callback has been called, the surrounding code (including the try { } catch(err) { } will have already exited. Throwing an error inside a node style callback will crash the process in most cases. If domains are enabled, they may intercept the thrown error; similarly, if a handler has been added to process.on('uncaughtException'), it will intercept the error.

JavaScript Errors

JavaScript errors typically denote that an API is being used incorrectly, or that there is a problem with the program as written.

Class: Error

A general error object. Unlike other error objects, Error instances do not denote any specific circumstance of why the error occurred. Errors capture a "stack trace" detailing the point in the program at which they were instantiated, and may provide a description of the error.

Note: Node.js will generate this class of error to encapsulate system errors as well as plain JavaScript errors.

new Error(message)

Instantiates a new Error object and sets its .message property to the provided message. Its .stack will represent the point in the program at which new Error was called. Stack traces are subject to V8's stack trace API. Stack traces only extend to the beginning of synchronous code execution, or a number of frames given by 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 program at which Error.captureStackTrace was called.

var myObject = {};

Error.captureStackTrace(myObject);

myObject.stack  // similar to `new Error().stack`

The first line of the trace, instead of being prefixed with ErrorType: message, will be the result of targetObject.toString().

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

This is useful for hiding implementation details of error generation from the end user. A common way of using this parameter is to pass the current Error constructor to it:


function MyError() {
  Error.captureStackTrace(this, MyError);
}

// without passing MyError to captureStackTrace, the MyError
// frame would should up in the .stack property. by passing
// the constructor, we omit that frame and all frames above it.
new MyError().stack

Error.stackTraceLimit

Property that determines the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The initial value is 10. It may be set to any valid JavaScript number, which will affect any stack trace captured after the value has been changed. If set to a non-number value, stack traces will not capture any frames and will report undefined on access.

error.message

A string of the value passed to Error() upon instantiation. The message will also appear in the first line of the stack trace of the error. Changing this property may not change the first line of the stack trace.

error.stack

A property that, when accessed, returns a string representing the point in the program at which this error was instantiated. An example stacktrace follows:

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 it is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site in the program 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 stacktraces:

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.

It is important to note that the string representing the stacktrace is only generated on access: it is lazily generated.

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 below.

Class: RangeError

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 be a numeric range, or outside the set of options for a given function parameter. An example:

require('net').connect(-1);  // throws RangeError, port should be > 0 && < 65536

Node.js will generate and throw RangeError instances immediately -- they are a form of argument validation.

Class: ReferenceError

A subclass of Error that indicates that an attempt is being made to access a variable that is not defined. Most commonly it indicates a typo, 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.

ReferenceError instances will have an .arguments member that is an array containing one element -- a string representing the variable that was not defined.

try {
  doesNotExist;
} catch(err) {
  err.arguments[0] === 'doesNotExist';
}

Unless the userland program is dynamically generating and running code, ReferenceErrors should always be considered a bug in the program, or its dependencies.

Class: SyntaxError

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
}

SyntaxErrors are unrecoverable from the context that created them – they may only be caught by other contexts.

Class: TypeError

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(function() { }); // throws TypeError, since it expected a string

Node.js will generate and throw TypeError instances immediately -- they are 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. These exceptions will always bring down the process. These are usually failed assert() checks or abort() calls in the C++ layer.

System Errors

System errors are generated in response to a program's runtime environment. Ideally, they represent operational errors that the program needs to be able to react to. They are 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 -- not full subclasses, but instead an error instance with added members.

Class: System Error

error.code

error.errno

A string representing the error code, which is always E followed by capital letters, and may be referenced in man 2 intro.

error.syscall

A string representing the syscall that failed.

Common System Errors

This list is not exhaustive, but enumerates many of the common system errors 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.

Commonly encountered when opening many files at once in parallel, especially on systems (in particular, OS X) 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; 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 appropriate 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 connected socket was not .end()'d appropriately.

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