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doc: improvements to errors.markdown copy 

General improvements to errors.markdown including
improved/revised examples

PR-URL: https://github.com/nodejs/node/pull/4454
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
process-exit-stdio-flushing
James M Snell 9 years ago
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      doc/api/errors.markdown

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@ -2,222 +2,257 @@
<!--type=misc-->
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][].
Applications running in Node.js will generally experience four categories of
errors:
- Standard JavaScript errors such as:
- [`EvalError`][]: thrown when a call to `eval()` fails.
- [`SyntaxError`][]: thrown in response to improper JavaScript language
syntax.
- [`RangeError`][]: thrown when a value is not within an expected range
- [`ReferenceError`][]: thrown when using undefined variables
- [`TypeError`][]: thrown when passing arguments of the wrong type
- [`URIError`][]: thrown when a global URI handling function is misused.
- System errors triggered by underlying operating system constraints such
as attempting to open a file that does not exist, attempting to send data
over a closed socket, etc;
- And User-specified errors triggered by application code.
- Assertion Errors are a special class of error that can be triggered whenever
Node.js detects an exceptional logic violation that should never occur. These
are raised typically by the `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
<!--type=misc-->
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.
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.
}
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.
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.
### Error events
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.
<!--type=misc-->
Errors that occur within _Asynchronous APIs_ may be reported in multiple ways:
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.
- 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.
```javascript
const net = require('net');
```
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);
});
const connection = net.connect('localhost');
connection.pipe(process.stdout);
```
// 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);
});
- 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.
connection.pipe(process.stdout);
```
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).
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:
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.
```javascript
const EventEmitter = require('events');
const EventEmitter = require('events');
const ee = new EventEmitter();
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'));
});
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.
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.
Developers must refer to the documentation for each method to determine
exactly how errors raised by those methods are propagated.
### Node style callbacks
### Node.js style callbacks
<!--type=misc-->
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:
```javascript
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:
```javascript
// 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;
Most asynchronous methods exposed by the Node.js core API follow an idiomatic
pattern 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 nodeStyleCallback(err, data) {
if (err) {
console.error('There was an error', err);
return;
}
console.log(data);
}
});
} 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
<!--type=misc-->
fs.readFile('/some/file/that/does-not-exist', nodeStyleCallback);
fs.readFile('/some/file/that/does-exist', nodeStyleCallback)
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 a Node.js 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) {
// This will not catch the throw!
console.log(err);
}
JavaScript errors typically denote that an API is being used incorrectly, or that
there is a problem with the program as written.
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.
### Class: Error
## Class: Error
<!--type=class-->
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.
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.
**Note**: Node.js will generate this class of error to encapsulate system
errors as well as plain JavaScript errors.
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)
### 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.
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.
#### Error.captureStackTrace(targetObject[, constructorOpt])
### 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.
a string representing the location in the code at which
`Error.captureStackTrace()` was called.
```javascript
var myObject = {};
Error.captureStackTrace(myObject);
myObject.stack // similar to `new Error().stack`
```
const 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.
message`, will be the result of calling `targetObject.toString()`.
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:
The optional `constructorOpt` argument accepts a function. If given, all frames
above `constructorOpt`, including `constructorOpt`, will be omitted from the
generated stack trace.
```javascript
The `constructorOpt` argument is useful for hiding implementation
details of error generation from an end user. For instance:
function MyError() {
Error.captureStackTrace(this, MyError);
}
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
// 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
#### 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)`).
Property that determines 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.
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.
If set to a non-number value, or set to a negative number, stack traces will
not capture any frames.
#### 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.
Returns the string description of 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.
const err = new Error('The message');
console.log(err.message);
// Prints: The message
#### 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:
Returns 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
@ -225,103 +260,107 @@ at which this error was instantiated. An example stacktrace follows:
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:
```javascript
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 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.
It is important to note that 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.
* `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 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
[below](#errors_system_errors).
System-level errors are generated as augmented `Error` instances, which are
detailed [below](#errors_system_errors).
### Class: RangeError
## 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:
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:
```javascript
require('net').connect(-1); // throws RangeError, port should be > 0 && < 65536
```
require('net').connect(-1);
// throws RangeError, port should be > 0 && < 65536
Node.js will generate and throw `RangeError` instances *immediately* -- they are a form
Node.js will generate and throw `RangeError` instances *immediately* as a form
of argument validation.
### Class: ReferenceError
## 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.
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.
```javascript
doesNotExist; // throws ReferenceError, doesNotExist is not a variable in this program.
```
While client code may generate and propagate these errors, in practice, only V8
will do so.
`ReferenceError` instances will have an `.arguments` member that is an array containing
one element -- a string representing the variable that was not defined.
doesNotExist;
// throws ReferenceError, doesNotExist is not a variable in this program.
```javascript
try {
doesNotExist;
} catch(err) {
err.arguments[0] === 'doesNotExist';
}
```
`ReferenceError` instances will have an `error.arguments` property whose value
is an array containing a single element: a string representing the variable
that was not defined.
Unless the userland program is dynamically generating and running code,
ReferenceErrors should always be considered a bug in the program, or its
dependencies.
const assert = require('assert');
try {
doesNotExist;
} catch(err) {
assert(err.arguments[0], 'doesNotExist');
}
Unless an application is dynamically generating and running code,
`ReferenceError` instances should always be considered a bug in the code
or its dependencies.
### Class: SyntaxError
## 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
@ -329,144 +368,129 @@ evaluation. Code evaluation may happen as a result of `eval`, `Function`,
`require`, or [vm][]. These errors are almost always indicative of a broken
program.
```javascript
try {
require('vm').runInThisContext('binary ! isNotOk');
} catch(err) {
// err will be a SyntaxError
}
```
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.
`SyntaxError` instances are unrecoverable in the context that created them –
they may only be caught by other contexts.
### Class: TypeError
## 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.
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.
```javascript
require('url').parse(function() { }); // throws TypeError, since it expected a string
```
require('url').parse(function() { });
// throws TypeError, since it expected a string
Node.js will generate and throw `TypeError` instances *immediately* -- they are a form
Node.js will generate and throw `TypeError` instances *immediately* as a form
of argument validation.
### Exceptions vs. Errors
## Exceptions vs. Errors
<!--type=misc-->
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.
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.
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
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][].
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 -- not full
subclasses, but instead an error instance with added members.
In Node.js, system errors are represented as augmented `Error` objects with
added properties.
### 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`.
Returns a string representing the error code, which is always `E` followed by
a sequence of capital letters, and may be referenced in `man 2 intro`.
The properties `error.code` and `error.errno` are aliases of one another and
return the same value.
#### error.syscall
A string representing the [syscall][] that failed.
Returns a string describing 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][online].
#### 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 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][online].
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.
- `EACCES` (Permission denied): An attempt was made to access a file in a way
forbidden by its file access permissions.
#### ENOENT: No such file or directory
- `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.
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.
- `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.
#### ENOTDIR: Not a directory
- `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.
A component of the given pathname existed, but was not a directory as expected.
Commonly raised by [`fs.readdir`][].
- `EEXIST` (File exists): An existing file was the target of an operation that
required that the target not exist.
#### ENOTEMPTY: Directory not empty
- `EISDIR` (Is a directory): An operation expected a file, but the given
pathname was a directory.
A directory with entries was the target of an operation that requires
an empty directory -- usually [`fs.unlink`][].
- `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, 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.
#### EPERM: Operation not permitted
- `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.
An attempt was made to perform an operation that requires appropriate
privileges.
- `ENOTDIR` (Not a directory): A component of the given pathname existed, but
was not a directory as expected. Commonly raised by [`fs.readdir`][].
#### EPIPE: Broken pipe
- `ENOTEMPTY` (Directory not empty): A directory with entries was the target
of an operation that requires an empty directory -- usually [`fs.unlink`][].
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.
- `EPERM` (Operation not permitted): An attempt was made to perform an
operation that requires elevated privileges.
#### ETIMEDOUT: Operation timed out
- `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.
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.
- `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.
[`Error`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
[`fs.readdir`]: fs.html#fs_fs_readdir_path_callback
@ -488,3 +512,7 @@ often a sign that a connected socket was not `.end()`'d appropriately.
[syscall]: http://man7.org/linux/man-pages/man2/syscall.2.html
[V8's stack trace API]: https://github.com/v8/v8/wiki/Stack-Trace-API
[vm]: vm.html
[`SyntaxError`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError
[`ReferenceError`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError
[`TypeError`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError
[`domain`]: domain.html

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