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<li><a href="#console_console">Console</a><ul>
<li><a href="#console_asynchronous_vs_synchronous_consoles">Asynchronous vs Synchronous Consoles</a></li>
<li><a href="#console_class_console">Class: Console</a><ul>
<li><a href="#console_new_console_stdout_stderr">new Console(stdout[, stderr])</a></li>
<li><a href="#console_console_assert_value_message">console.assert(value[, message][, ...])</a></li>
<li><a href="#console_console_dir_obj_options">console.dir(obj[, options])</a></li>
<li><a href="#console_console_error_data">console.error([data][, ...])</a></li>
<li><a href="#console_console_info_data">console.info([data][, ...])</a></li>
<li><a href="#console_console_log_data">console.log([data][, ...])</a></li>
<li><a href="#console_console_time_label">console.time(label)</a></li>
<li><a href="#console_console_timeend_label">console.timeEnd(label)</a></li>
<li><a href="#console_console_trace_message">console.trace(message[, ...])</a></li>
<li><a href="#console_console_warn_data">console.warn([data][, ...])</a></li>
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<h1>Console<span><a class="mark" href="#console_console" id="console_console">#</a></span></h1>
<pre class="api_stability_2">Stability: 2 - Stable</pre><p>The <code>console</code> module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
</p>
<p>The module exports two specific components:
</p>
<ul>
<li>A <code>Console</code> class with methods such as <code>console.log()</code>, <code>console.error()</code> and
<code>console.warn()</code> that can be used to write to any Node.js stream.</li>
<li>A global <code>console</code> instance configured to write to <code>stdout</code> and <code>stderr</code>.
Because this object is global, it can be used without calling
<code>require('console')</code>.</li>
</ul>
<p>Example using the global <code>console</code>:
</p>
<pre><code>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</code></pre>
<p>Example using the <code>Console</code> class:
</p>
<pre><code>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 ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err</code></pre>
<p>While the API for the <code>Console</code> class is designed fundamentally around the
Web browser <code>console</code> object, the <code>Console</code> is Node.js is <em>not</em> intended to
duplicate the browsers functionality exactly.
</p>
<h2>Asynchronous vs Synchronous Consoles<span><a class="mark" href="#console_asynchronous_vs_synchronous_consoles" id="console_asynchronous_vs_synchronous_consoles">#</a></span></h2>
<p>The console functions are synchronous when the destination is a terminal or
a file (to avoid lost messages in case of premature exit) and asynchronous
when the destination is a pipe (to avoid blocking for long periods of time).
</p>
<p>In the following example, stdout is non-blocking while stderr is blocking:
</p>
<pre><code>$ node script.js 2> error.log | tee info.log</code></pre>
<p>Typically, the distinction between blocking/non-blocking is not important
unless an application is logging significant amounts of data. High volume
logging <em>should</em> use a <code>Console</code> instance that writes to a pipe.
</p>
<h2>Class: Console<span><a class="mark" href="#console_class_console" id="console_class_console">#</a></span></h2>
<!--type=class-->
<p>The <code>Console</code> class can be used to create a simple logger with configurable
output streams and can be accessed using either <code>require('console').Console</code>
or <code>console.Console</code>:
</p>
<pre><code>const Console = require('console').Console;
const Console = console.Console;</code></pre>
<h3>new Console(stdout[, stderr])<span><a class="mark" href="#console_new_console_stdout_stderr" id="console_new_console_stdout_stderr">#</a></span></h3>
<p>Creates a new <code>Console</code> by passing one or two writable stream instances.
<code>stdout</code> is a writable stream to print log or info output. <code>stderr</code>
is used for warning or error output. If <code>stderr</code> isn't passed, the warning
and error output will be sent to the <code>stdout</code>.
</p>
<pre><code>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</code></pre>
<p>The global <code>console</code> is a special <code>Console</code> whose output is sent to
<code>process.stdout</code> and <code>process.stderr</code>. It is equivalent to calling:
</p>
<pre><code>new Console(process.stdout, process.stderr);</code></pre>
<h3>console.assert(value[, message][, ...])<span><a class="mark" href="#console_console_assert_value_message" id="console_console_assert_value_message">#</a></span></h3>
<p>A simple assertion test that verifies whether <code>value</code> is truthy. If it is not,
an <code>AssertionError</code> is throw. If provided, the error <code>message</code> is formatted
using <a href="util.html#util_util_format_format"><code>util.format()</code></a> and used as the error message.
</p>
<pre><code>console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work</code></pre>
<h3>console.dir(obj[, options])<span><a class="mark" href="#console_console_dir_obj_options" id="console_console_dir_obj_options">#</a></span></h3>
<p>Uses <a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> on <code>obj</code> and prints the resulting string to stdout.
This function bypasses any custom <code>inspect()</code> function defined on <code>obj</code>. An
optional <code>options</code> object may be passed that alters certain aspects of the
formatted string:
</p>
<ul>
<li><p><code>showHidden</code> - if <code>true</code> then the object's non-enumerable and symbol
properties will be shown too. Defaults to <code>false</code>.</p>
</li>
<li><p><code>depth</code> - tells <code>inspect</code> how many times to recurse while formatting the
object. This is useful for inspecting large complicated objects. Defaults to
<code>2</code>. To make it recurse indefinitely, pass <code>null</code>.</p>
</li>
<li><p><code>colors</code> - if <code>true</code>, then the output will be styled with ANSI color codes.
Defaults to <code>false</code>. Colors are customizable; see
[customizing <code>util.inspect()</code> colors][].</p>
</li>
</ul>
<h3>console.error([data][, ...])<span><a class="mark" href="#console_console_error_data" id="console_console_error_data">#</a></span></h3>
<p>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 <code>printf()</code> (the arguments are all passed to
<a href="util.html#util_util_format_format"><code>util.format()</code></a>).
</p>
<pre><code>const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr</code></pre>
<p>If formatting elements (e.g. <code>%d</code>) are not found in the first string then
<a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> is called on each argument and the resulting string
values are concatenated. See <a href="util.html#util_util_format_format"><code>util.format()</code></a> for more information.
</p>
<h3>console.info([data][, ...])<span><a class="mark" href="#console_console_info_data" id="console_console_info_data">#</a></span></h3>
<p>The <code>console.info()</code> function is an alias for <a href="#console_console_log_data"><code>console.log()</code></a>.
</p>
<h3>console.log([data][, ...])<span><a class="mark" href="#console_console_log_data" id="console_console_log_data">#</a></span></h3>
<p>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 <code>printf()</code> (the arguments are all passed to
<a href="util.html#util_util_format_format"><code>util.format()</code></a>).
</p>
<pre><code>var count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count: ', count);
// Prints: count: 5, to stdout</code></pre>
<p>If formatting elements (e.g. <code>%d</code>) are not found in the first string then
<a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> is called on each argument and the resulting string
values are concatenated. See <a href="util.html#util_util_format_format"><code>util.format()</code></a> for more information.
</p>
<h3>console.time(label)<span><a class="mark" href="#console_console_time_label" id="console_console_time_label">#</a></span></h3>
<p>Used to calculate the duration of a specific operation. To start a timer, call
the <code>console.time()</code> method, giving it a unique <code>label</code> as the only parameter. To stop the
timer, and to get the elapsed time in milliseconds, just call the
<a href="#console_console_timeend_label"><code>console.timeEnd()</code></a> method, again passing the
timer's unique <code>label</code> as the parameter.
</p>
<h3>console.timeEnd(label)<span><a class="mark" href="#console_console_timeend_label" id="console_console_timeend_label">#</a></span></h3>
<p>Stops a timer that was previously started by calling <a href="#console_console_time_label"><code>console.time()</code></a> and
prints the result to stdout:
</p>
<pre><code>console.time('100-elements');
for (var i = 0; i < 100; i++) {
;
}
console.timeEnd('100-elements');
// prints 100-elements: 262ms</code></pre>
<h3>console.trace(message[, ...])<span><a class="mark" href="#console_console_trace_message" id="console_console_trace_message">#</a></span></h3>
<p>Prints to stderr the string <code>'Trace :'</code>, followed by the <a href="util.html#util_util_format_format"><code>util.format()</code></a>
formatted message and stack trace to the current position in the code.
</p>
<pre><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)</code></pre>
<h3>console.warn([data][, ...])<span><a class="mark" href="#console_console_warn_data" id="console_console_warn_data">#</a></span></h3>
<p>The <code>console.warn()</code> function is an alias for <a href="#console_console_error_data"><code>console.error()</code></a>.
</p>
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