# 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`:

```js
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:

```js
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 usually 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.

Additionally, console functions are blocking when outputting to TTYs
(terminals) on OS X as a workaround for the OS's very small, 1kb buffer size.
This is to prevent interleaving between `stdout` and `stderr`.

## Class: Console

<!--type=class-->

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`:

```js
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`.

```js
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:

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

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

```js
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][web-api-assert].*

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.

```js
'use strict';

// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.setPrototypeOf({
  assert(assertion, message, ...args) {
    try {
      console.assert(assertion, message, ...args);
    } catch (err) {
      console.error(err.stack);
    }
  }
}, console);

module.exports = myConsole;
```

This can then be used as a direct replacement for the built in console:

```js
const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');
```

### console.dir(obj[, options])
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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][].

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

```js
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][, ...args])
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The `console.info()` function is an alias for [`console.log()`][].

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

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

```js
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.*

### console.trace(message[, ...args])
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Prints to `stderr` the string `'Trace :'`, followed by the [`util.format()`][]
formatted message and stack trace to the current position in the code.

```js
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])
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The `console.warn()` function is an alias for [`console.error()`][].

[`console.error()`]: #console_console_error_data_args
[`console.log()`]: #console_console_log_data_args
[`console.time()`]: #console_console_time_label
[`console.timeEnd()`]: #console_console_timeend_label
[`process.stderr`]: process.html#process_process_stderr
[`process.stdout`]: process.html#process_process_stdout
[`util.format()`]: util.html#util_util_format_format_args
[`util.inspect()`]: util.html#util_util_inspect_object_options
[customizing `util.inspect()` colors]: util.html#util_customizing_util_inspect_colors
[web-api-assert]: https://developer.mozilla.org/en-US/docs/Web/API/console/assert