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 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.
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
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)
Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label
. Use the same label
when you call console.timeEnd()
to stop the timer and output the elapsed time in milliseconds to stdout. Timer durations are accurate to the sub-millisecond.
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: 225.438ms
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()
.