nodejs/doc/api/console.markdown

Console

Stability: 2 - Stable

The module defines a Console class and exports a console object.

The console object is a special instance of Console whose output is sent to stdout or stderr.

For ease of use, console is defined as a global object and can be used directly without require.

console

• {Object}

For printing to stdout and stderr. Similar to the console object functions provided by most web browsers, here the output is sent to stdout or stderr.

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 it's a pipe (to avoid blocking for long periods of time).

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

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

In daily use, the blocking/non-blocking dichotomy is not something you should worry about unless you log huge amounts of data.

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

Prints to stdout with newline. This function can take multiple arguments in a printf()-like way. Example:

var count = 5;
console.log('count: %d', count);
// prints 'count: 5'

If formatting elements are not found in the first string then util.inspect is used on each argument. See util.format() for more information.

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

Same as console.log.

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

Same as console.log but prints to stderr.

console.warn([data][, ...])

Same as console.error.

console.dir(obj[, options])

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

console.time(timerName)

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

console.timeEnd(timerName)

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

Example:

console.time('100-elements');
for (var i = 0; i < 100; i++) {
  ;
}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

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

Print to stderr 'Trace :', followed by the formatted message and stack trace to the current position.

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

Similar to assert.ok(), but the error message is formatted as util.format(message...).

Class: Console

Use require('console').Console or console.Console to access this class.

var Console = require('console').Console;
var Console = console.Console;

You can use Console class to custom simple logger like console, but with different output streams.

new Console(stdout[, stderr])

Create 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.

var output = fs.createWriteStream('./stdout.log');
var errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
var 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:

new Console(process.stdout, process.stderr);