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Node.js v4.7.1 Documentation
Table of Contents
Console#
Stability: 2 - Stable
The console
module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
- A
Console
class with methods such asconsole.log()
,console.error()
andconsole.warn()
that can be used to write to any Node.js stream. - A global
console
instance configured to write tostdout
andstderr
. Because this object is global, it can be used without callingrequire('console')
.
Example using the global console
:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
Example using the Console
class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
While the API for the Console
class is designed fundamentally around the
browser console
object, the Console
in Node.js is not intended to
duplicate the browser's functionality exactly.
Asynchronous vs Synchronous Consoles#
The console functions are usually asynchronous unless the destination is a file. Disks are fast and operating systems normally employ write-back caching; it should be a very rare occurrence indeed that a write blocks, but it is possible.
Additionally, console functions are blocking when outputting to TTYs
(terminals) on OS X as a workaround for the OS's very small, 1kb buffer size.
This is to prevent interleaving between stdout
and stderr
.
Class: Console#
The Console
class can be used to create a simple logger with configurable
output streams and can be accessed using either require('console').Console
or console.Console
:
const Console = require('console').Console;
const Console = console.Console;
new Console(stdout[, stderr])#
Creates a new Console
by passing one or two writable stream instances.
stdout
is a writable stream to print log or info output. stderr
is used for warning or error output. If stderr
isn't passed, warning and error
output will be sent to stdout
.
const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, errorOutput);
// use it like console
var count = 5;
logger.log('count: %d', count);
// in stdout.log: count 5
The global console
is a special Console
whose output is sent to
process.stdout
and process.stderr
. It is equivalent to calling:
new Console(process.stdout, process.stderr);
console.assert(value[, message][, ...])#
A simple assertion test that verifies whether value
is truthy. If it is not,
an AssertionError
is thrown. If provided, the error message
is formatted
using util.format()
and used as the error message.
console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work
Note: the console.assert()
method is implemented differently in Node.js
than the console.assert()
method available in browsers.
Specifically, in browsers, calling console.assert()
with a falsy
assertion will cause the message
to be printed to the console without
interrupting execution of subsequent code. In Node.js, however, a falsy
assertion will cause an AssertionError
to be thrown.
Functionality approximating that implemented by browsers can be implemented
by extending Node.js' console
and overriding the console.assert()
method.
In the following example, a simple module is created that extends and overrides
the default behavior of console
in Node.js.
'use strict';
// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.setPrototypeOf({
assert(assertion, message, ...args) {
try {
console.assert(assertion, message, ...args);
} catch (err) {
console.error(err.stack);
}
}
}, console);
module.exports = myConsole;
This can then be used as a direct replacement for the built in console:
const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');
console.dir(obj[, options])#
Uses util.inspect()
on obj
and prints the resulting string to stdout
.
This function bypasses any custom inspect()
function defined on obj
. An
optional options
object may be passed to alter certain aspects of the
formatted string:
showHidden
- iftrue
then the object's non-enumerable and symbol properties will be shown too. Defaults tofalse
.depth
- tellsutil.inspect()
how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to2
. To make it recurse indefinitely, passnull
.colors
- iftrue
, then the output will be styled with ANSI color codes. Defaults tofalse
. Colors are customizable; see customizingutil.inspect()
colors.
console.error([data][, ...])#
Prints to stderr
with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3) (the arguments are all passed to
util.format()
).
const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr
If formatting elements (e.g. %d
) are not found in the first string then
util.inspect()
is called on each argument and the resulting string
values are concatenated. See util.format()
for more information.
console.info([data][, ...])#
The console.info()
function is an alias for console.log()
.
console.log([data][, ...])#
Prints to stdout
with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3) (the arguments are all passed to
util.format()
).
var count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
If formatting elements (e.g. %d
) are not found in the first string then
util.inspect()
is called on each argument and the resulting string
values are concatenated. See util.format()
for more information.
console.time(label)#
Used to calculate the duration of a specific operation. To start a timer, call
the console.time()
method, giving it a unique label
as the only parameter. To stop the
timer, and to get the elapsed time in milliseconds, just call the
console.timeEnd()
method, again passing the
timer's unique label
as the parameter.
console.timeEnd(label)#
Stops a timer that was previously started by calling console.time()
and
prints the result to stdout:
console.time('100-elements');
for (var i = 0; i < 100; i++) {
;
}
console.timeEnd('100-elements');
// prints 100-elements: 262ms
console.trace(message[, ...])#
Prints to stderr
the string 'Trace :'
, followed by the util.format()
formatted message and stack trace to the current position in the code.
console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
// Trace: Show me
// at repl:2:9
// at REPLServer.defaultEval (repl.js:248:27)
// at bound (domain.js:287:14)
// at REPLServer.runBound [as eval] (domain.js:300:12)
// at REPLServer.<anonymous> (repl.js:412:12)
// at emitOne (events.js:82:20)
// at REPLServer.emit (events.js:169:7)
// at REPLServer.Interface._onLine (readline.js:210:10)
// at REPLServer.Interface._line (readline.js:549:8)
// at REPLServer.Interface._ttyWrite (readline.js:826:14)
console.warn([data][, ...])#
The console.warn()
function is an alias for console.error()
.