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:
Console
class with methods such as console.log()
, console.error()
and console.warn()
that can be used to write to any Node.js stream.console
instance configured to write to process.stdout
and process.stderr
. The global console
can be used without calling require('console')
.Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.
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
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;
Creates a new Console
with 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
is not provided, stdout
is used for stderr
.
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 const 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);
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.create(console, { assert: { value(assertion, message, ...args) { try { console.assert(assertion, message, ...args); } catch (err) { console.error(err.stack); } }, configurable: true, enumerable: true, writable: true, }, }); 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');
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
- if true
then the object's non-enumerable and symbol properties will be shown too. Defaults to false
.
depth
- tells util.inspect()
how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to 2
. To make it recurse indefinitely, pass null
.
colors
- if true
, then the output will be styled with ANSI color codes. Defaults to false
. Colors are customizable; see customizing util.inspect()
colors.
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.
The console.info()
function is an alias for console.log()
.
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()
).
const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout
See util.format()
for more information.
label
<string>
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.
label
<string>
Stops a timer that was previously started by calling console.time()
and prints the result to stdout
:
console.time('100-elements'); for (let i = 0; i < 100; i++) ; console.timeEnd('100-elements'); // prints 100-elements: 225.438ms
Note: As of Node.js v6.0.0, console.timeEnd()
deletes the timer to avoid leaking it. On older versions, the timer persisted. This allowed console.timeEnd()
to be called multiple times for the same label. This functionality was unintended and is no longer supported.
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)
The console.warn()
function is an alias for console.error()
.
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https://nodejs.org/dist/latest-v6.x/docs/api/console.html