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io.js v3.3.1 Documentation
Table of Contents
- Errors
- JavaScript Errors
- System Errors
- Class: System Error
- Common System Errors
- EPERM: Operation not permitted
- ENOENT: No such file or directory
- EACCES: Permission denied
- EEXIST: File exists
- ENOTDIR: Not a directory
- EISDIR: Is a directory
- EMFILE: Too many open files in system
- EPIPE: Broken pipe
- EADDRINUSE: Address already in use
- ECONNRESET: Connection reset by peer
- ECONNREFUSED: Connection refused
- ENOTEMPTY: Directory not empty
- ETIMEDOUT: Operation timed out
- Error Propagation and Interception
Errors#
Errors generated by io.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.
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: io.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.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:
var 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 io.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.
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.
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
io.js will generate and throw RangeError instances immediately -- they are a form of argument validation.
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
io.js will generate and throw TypeError 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.
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 inherit from
Error
, all exceptions thrown by io.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 io.js, system errors are represented as augmented Error objects -- not full subclasses, but instead an error instance with added members.
Class: System Error#
error.syscall#
A string representing the syscall that failed.
error.errno#
error.code#
A string representing the error code, which is always E
followed by capital
letters, and may be referenced in man 2 intro
.
Common System Errors#
This list is not exhaustive, but enumerates many of the common system errors when writing a io.js program. An exhaustive list may be found here.
EPERM: Operation not permitted#
An attempt was made to perform an operation that requires appropriate privileges.
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.
EACCES: Permission denied#
An attempt was made to access a file in a way forbidden by its file access permissions.
EEXIST: File exists#
An existing file was the target of an operation that required that the target not exist.
ENOTDIR: Not a directory#
A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir.
EISDIR: Is a directory#
An operation expected a file, but the given pathname was a directory.
EMFILE: Too many open files in system#
Maxiumum 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 io.js process.
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.
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.
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.
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.
ENOTEMPTY: Directory not empty#
A directory with entries was the target of an operation that requires an empty directory -- usually fs.unlink.
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.
Error Propagation and Interception#
All io.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.
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:
var 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', function(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:
var fs = require('fs');
try {
fs.readFile('/some/file/that/does-not-exist', function(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.
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.
var net = require('net');
var connection = net.connect('localhost');
// adding an "error" event handler to a stream:
connection.on('error', function(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 io.js -- even user created event emitters and streams will throw errors when no error handlers are attached. An example:
var events = require('events');
var ee = new events.EventEmitter;
setImmediate(function() {
// 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.