Console | Node.js v23.11.0 Documentation (original) (raw)

Source Code: lib/console.js

The node: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:

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. Programs that desire to depend on the synchronous / asynchronous behavior of the console functions should first figure out the nature of console's backing stream. This is because the stream is dependent on the underlying platform and standard stream configuration of the current process. 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 message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3

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 ``

Class: Console#

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('node:console').Consoleor console.Console (or their destructured counterparts):

import { Console } from 'node:console'; const { Console } = require('node:console');

const { Console } = console;

new Console(stdout[, stderr][, ignoreErrors])#

new Console(options)#

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.

`import { createWriteStream } from 'node:fs'; import { Console } from 'node:console'; // Alternatively // const { Console } = console;

const output = createWriteStream('./stdout.log'); const errorOutput = createWriteStream('./stderr.log'); // Custom simple logger const logger = new Console({ stdout: output, stderr: errorOutput }); // use it like console const count = 5; logger.log('count: %d', count); // In stdout.log: count 5 const fs = require('node:fs'); const { Console } = require('node:console'); // Alternatively // const { Console } = console;

const output = fs.createWriteStream('./stdout.log'); const errorOutput = fs.createWriteStream('./stderr.log'); // Custom simple logger const logger = new Console({ stdout: output, stderr: 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 toprocess.stdout and process.stderr. It is equivalent to calling:

new Console({ stdout: process.stdout, stderr: process.stderr });

console.assert(value[, ...message])#

console.assert() writes a message if value is falsy or omitted. It only writes a message and does not otherwise affect execution. The output always starts with "Assertion failed". If provided, message is formatted usingutil.format().

If value is truthy, nothing happens.

`console.assert(true, 'does nothing');

console.assert(false, 'Whoops %s work', 'didn't'); // Assertion failed: Whoops didn't work

console.assert(); // Assertion failed`

console.clear()#

Added in: v8.3.0

When stdout is a TTY, calling console.clear() will attempt to clear the TTY. When stdout is not a TTY, this method does nothing.

The specific operation of console.clear() can vary across operating systems and terminal types. For most Linux operating systems, console.clear()operates similarly to the clear shell command. On Windows, console.clear()will clear only the output in the current terminal viewport for the Node.js binary.

console.count([label])#

Added in: v8.3.0

Maintains an internal counter specific to label and outputs to stdout the number of times console.count() has been called with the given label.

`> console.count() default: 1 undefined

console.count('default') default: 2 undefined console.count('abc') abc: 1 undefined console.count('xyz') xyz: 1 undefined console.count('abc') abc: 2 undefined console.count() default: 3 undefined `

console.countReset([label])#

Added in: v8.3.0

Resets the internal counter specific to label.

`> console.count('abc'); abc: 1 undefined

console.countReset('abc'); undefined console.count('abc'); abc: 1 undefined `

console.debug(data[, ...args])#

The console.debug() function is an alias for console.log().

console.dir(obj[, options])#

Added in: v0.1.101

Uses util.inspect() on obj and prints the resulting string to stdout. This function bypasses any custom inspect() function defined on obj.

console.dirxml(...data)#

This method calls console.log() passing it the arguments received. This method does not produce any XML formatting.

console.error([data][, ...args])#

Added in: v0.1.100

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 toutil.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 thenutil.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.group([...label])#

Added in: v8.5.0

Increases indentation of subsequent lines by spaces for groupIndentationlength.

If one or more labels are provided, those are printed first without the additional indentation.

console.groupCollapsed()#

Added in: v8.5.0

An alias for console.group().

console.groupEnd()#

Added in: v8.5.0

Decreases indentation of subsequent lines by spaces for groupIndentationlength.

console.info([data][, ...args])#

Added in: v0.1.100

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Added in: v0.1.100

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 toutil.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.

console.table(tabularData[, properties])#

Added in: v10.0.0

Try to construct a table with the columns of the properties of tabularData(or use properties) and rows of tabularData and log it. Falls back to just logging the argument if it can't be parsed as tabular.

`// These can't be parsed as tabular data console.table(Symbol()); // Symbol()

console.table(undefined); // undefined

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]); // ┌─────────┬─────┬─────┐ // │ (index) │ a │ b │ // ├─────────┼─────┼─────┤ // │ 0 │ 1 │ 'Y' │ // │ 1 │ 'Z' │ 2 │ // └─────────┴─────┴─────┘

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']); // ┌─────────┬─────┐ // │ (index) │ a │ // ├─────────┼─────┤ // │ 0 │ 1 │ // │ 1 │ 'Z' │ // └─────────┴─────┘`

console.time([label])#

Added in: v0.1.104

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 callingconsole.timeEnd() to stop the timer and output the elapsed time in suitable time units to stdout. For example, if the elapsed time is 3869ms, console.timeEnd() displays "3.869s".

console.timeEnd([label])#

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('bunch-of-stuff'); // Do a bunch of stuff. console.timeEnd('bunch-of-stuff'); // Prints: bunch-of-stuff: 225.438ms

console.timeLog([label][, ...data])#

Added in: v10.7.0

For a timer that was previously started by calling console.time(), prints the elapsed time and other data arguments to stdout:

console.time('process'); const value = expensiveProcess1(); // Returns 42 console.timeLog('process', value); // Prints "process: 365.227ms 42". doExpensiveProcess2(value); console.timeEnd('process');

console.trace([message][, ...args])#

Added in: v0.1.104

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][, ...args])#

Added in: v0.1.100

The console.warn() function is an alias for console.error().

Inspector only methods#

The following methods are exposed by the V8 engine in the general API but do not display anything unless used in conjunction with the inspector(--inspect flag).

console.profile([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. Theconsole.profile() method starts a JavaScript CPU profile with an optional label until console.profileEnd() is called. The profile is then added to the Profile panel of the inspector.

console.profile('MyLabel'); // Some code console.profileEnd('MyLabel'); // Adds the profile 'MyLabel' to the Profiles panel of the inspector.

console.profileEnd([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. Stops the current JavaScript CPU profiling session if one has been started and prints the report to the Profiles panel of the inspector. Seeconsole.profile() for an example.

If this method is called without a label, the most recently started profile is stopped.

console.timeStamp([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. Theconsole.timeStamp() method adds an event with the label 'label' to theTimeline panel of the inspector.