nodejs/doc/api/console.markdown

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 as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
• A global console instance configured to write to stdout and stderr. Because this object is global, it can be used without calling require('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 Web browser console object, the Console is Node.js is not intended to duplicate the browsers functionality exactly.

Asynchronous vs Synchronous Consoles

The console functions are synchronous when the destination is a terminal or a file (to avoid lost messages in case of premature exit) and asynchronous when the destination is a pipe (to avoid blocking for long periods of time).

In the following example, stdout is non-blocking while stderr is blocking:

$ node script.js 2> error.log | tee info.log

Typically, the distinction between blocking/non-blocking is not important unless an application is logging significant amounts of data. High volume logging should use a Console instance that writes to a pipe.

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, the warning and error output will be sent to the 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 throw. 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 that alters 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 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][].

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() (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() (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().