<!doctype html> <html lang="en"> <head> <meta charset="utf-8"> <title>Console | Node.js v6.17.1 Documentation</title> <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic"> <link rel="stylesheet" href="assets/style.css"> <link rel="stylesheet" href="assets/sh.css"> <link rel="canonical" href="https://nodejs.org/api/console.html"> </head> <body class="alt apidoc" id="api-section-console"> <div id="content" class="clearfix"> <div id="column2" class="interior"> <div id="intro" class="interior"> <a href="/" title="Go back to the home page"> Node.js </a> </div> <ul> <li><a class="nav-documentation" href="documentation.html">About these Docs</a></li> <li><a class="nav-synopsis" href="synopsis.html">Usage & Example</a></li> </ul> <div class="line"></div> <ul> <li><a class="nav-assert" href="assert.html">Assertion Testing</a></li> <li><a class="nav-buffer" href="buffer.html">Buffer</a></li> <li><a class="nav-addons" href="addons.html">C++ Addons</a></li> 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href="https://nodejs.org/docs/latest-v9.x/api/console.html">9.x</a></li> <li><a href="https://nodejs.org/docs/latest-v8.x/api/console.html">8.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v7.x/api/console.html">7.x</a></li> <li><a href="https://nodejs.org/docs/latest-v6.x/api/console.html">6.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v5.x/api/console.html">5.x</a></li> <li><a href="https://nodejs.org/docs/latest-v4.x/api/console.html">4.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v0.12.x/api/console.html">0.12.x</a></li> <li><a href="https://nodejs.org/docs/latest-v0.10.x/api/console.html">0.10.x</a></li></ol> </li> </ul> </div> <hr> </header> <div id="toc"> <h2>Table of Contents</h2> <ul> <li><span class="stability_2"><a href="#console_console">Console</a></span><ul> <li><span class="stability_undefined"><a href="#console_class_console">Class: Console</a></span><ul> <li><span class="stability_undefined"><a href="#console_new_console_stdout_stderr">new Console(stdout[, stderr])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_assert_value_message_args">console.assert(value[, message][, ...args])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_clear">console.clear()</a></span></li> <li><span class="stability_undefined"><a href="#console_console_count_label">console.count([label])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_countreset_label_default">console.countReset([label = 'default'])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_dir_obj_options">console.dir(obj[, options])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_error_data_args">console.error([data][, ...args])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_info_data_args">console.info([data][, ...args])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_log_data_args">console.log([data][, ...args])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_time_label">console.time(label)</a></span></li> <li><span class="stability_undefined"><a href="#console_console_timeend_label">console.timeEnd(label)</a></span></li> <li><span class="stability_undefined"><a href="#console_console_trace_message_args">console.trace(message[, ...args])</a></span></li> <li><span class="stability_undefined"><a href="#console_console_warn_data_args">console.warn([data][, ...args])</a></span></li> </ul> </li> </ul> </li> </ul> </div> <div id="apicontent"> <h1>Console<span><a class="mark" href="#console_console"id="console_console">#</a></span></h1> <!--introduced_in=v0.10.13--> <div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><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 <a href="process.html#process_process_stdout"><code>process.stdout</code></a> and <a href="process.html#process_process_stderr"><code>process.stderr</code></a>. The global <code>console</code> can be used without calling <code>require('console')</code>.</li> </ul> <p><strong><em>Warning</em></strong>: 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. See the <a href="process.html#process_a_note_on_process_i_o">note on process I/O</a> for more information.</p> <p>Example using the global <code>console</code>:</p> <pre><code class="lang-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 </code></pre> <p>Example using the <code>Console</code> class:</p> <pre><code class="lang-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 </code></pre> <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 class="lang-js">const Console = require('console').Console; </code></pre> <pre><code class="lang-js">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> with 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> is not provided, <code>stdout</code> is used for <code>stderr</code>.</p> <pre><code class="lang-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 </code></pre> <p>The global <code>console</code> is a special <code>Console</code> whose output is sent to <a href="process.html#process_process_stdout"><code>process.stdout</code></a> and <a href="process.html#process_process_stderr"><code>process.stderr</code></a>. It is equivalent to calling:</p> <pre><code class="lang-js">new Console(process.stdout, process.stderr); </code></pre> <h3>console.assert(value[, message][, ...args])<span><a class="mark" href="#console_console_assert_value_message_args"id="console_console_assert_value_message_args">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.101</span> </div><p>A simple assertion test that verifies whether <code>value</code> is truthy. If it is not, an <code>AssertionError</code> is thrown. If provided, the error <code>message</code> is formatted using <a href="util.html#util_util_format_format_args"><code>util.format()</code></a> and used as the error message.</p> <pre><code class="lang-js">console.assert(true, 'does nothing'); // OK console.assert(false, 'Whoops %s', 'didn\'t work'); // AssertionError: Whoops didn't work </code></pre> <p><em>Note: the <code>console.assert()</code> method is implemented differently in Node.js than the <code>console.assert()</code> method <a href="https://developer.mozilla.org/en-US/docs/Web/API/console/assert">available in browsers</a>.</em></p> <p>Specifically, in browsers, calling <code>console.assert()</code> with a falsy assertion will cause the <code>message</code> to be printed to the console without interrupting execution of subsequent code. In Node.js, however, a falsy assertion will cause an <code>AssertionError</code> to be thrown.</p> <p>Functionality approximating that implemented by browsers can be implemented by extending Node.js' <code>console</code> and overriding the <code>console.assert()</code> method.</p> <p>In the following example, a simple module is created that extends and overrides the default behavior of <code>console</code> in Node.js.</p> <pre><code class="lang-js">'use strict'; // Creates a simple extension of console with a // new impl for assert without monkey-patching. const myConsole = Object.create(console, { assert: { value(assertion, message, ...args) { try { console.assert(assertion, message, ...args); } catch (err) { console.error(err.stack); } }, configurable: true, enumerable: true, writable: true, }, }); module.exports = myConsole; </code></pre> <p>This can then be used as a direct replacement for the built in console:</p> <pre><code class="lang-js">const console = require('./myConsole'); console.assert(false, 'this message will print, but no error thrown'); console.log('this will also print'); </code></pre> <h3>console.clear()<span><a class="mark" href="#console_console_clear"id="console_console_clear">#</a></span></h3> <div class="api_metadata"> <span>Added in: v6.13.0</span> </div><p>When <code>stdout</code> is a TTY, calling <code>console.clear()</code> will attempt to clear the TTY. When <code>stdout</code> is not a TTY, this method does nothing.</p> <p><em>Note</em>: The specific operation of <code>console.clear()</code> can vary across operating systems and terminal types. For most Linux operating systems, <code>console.clear()</code> operates similarly to the <code>clear</code> shell command. On Windows, <code>console.clear()</code> will clear only the output in the current terminal viewport for the Node.js binary.</p> <h3>console.count([label])<span><a class="mark" href="#console_console_count_label"id="console_console_count_label">#</a></span></h3> <div class="api_metadata"> <span>Added in: v6.13.0</span> </div><ul> <li><code>label</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The display label for the counter. Defaults to <code>'default'</code>.</li> </ul> <p>Maintains an internal counter specific to <code>label</code> and outputs to <code>stdout</code> the number of times <code>console.count()</code> has been called with the given <code>label</code>.</p> <!-- eslint-skip --> <pre><code class="lang-js">> 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 > </code></pre> <h3>console.countReset([label = 'default'])<span><a class="mark" href="#console_console_countreset_label_default"id="console_console_countreset_label_default">#</a></span></h3> <div class="api_metadata"> <span>Added in: v6.13.0</span> </div><ul> <li><code>label</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The display label for the counter. Defaults to <code>'default'</code>.</li> </ul> <p>Resets the internal counter specific to <code>label</code>.</p> <!-- eslint-skip --> <pre><code class="lang-js">> console.count('abc'); abc: 1 undefined > console.countReset('abc'); undefined > console.count('abc'); abc: 1 undefined > </code></pre> <!-- eslint-enable --> <h3>console.dir(obj[, options])<span><a class="mark" href="#console_console_dir_obj_options"id="console_console_dir_obj_options">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.101</span> </div><ul> <li><code>obj</code> <span class="type"><any></span></li> <li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a><ul> <li><code>showHidden</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> <li><code>depth</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> <li><code>colors</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> </ul> </li> </ul> <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 <code>stdout</code>. This function bypasses any custom <code>inspect()</code> function defined on <code>obj</code>. An optional <code>options</code> object may be passed to alter 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 <a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> 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 <a href="util.html#util_customizing_util_inspect_colors">customizing <code>util.inspect()</code> colors</a>.</p> </li> </ul> <h3>console.error([data][, ...args])<span><a class="mark" href="#console_console_error_data_args"id="console_console_error_data_args">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.100</span> </div><p>Prints to <code>stderr</code> with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to <a href="http://man7.org/linux/man-pages/man3/printf.3.html">printf(3)</a> (the arguments are all passed to <a href="util.html#util_util_format_format_args"><code>util.format()</code></a>).</p> <pre><code class="lang-js">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_args"><code>util.format()</code></a> for more information.</p> <h3>console.info([data][, ...args])<span><a class="mark" href="#console_console_info_data_args"id="console_console_info_data_args">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.100</span> </div><p>The <code>console.info()</code> function is an alias for <a href="#console_console_log_data_args"><code>console.log()</code></a>.</p> <h3>console.log([data][, ...args])<span><a class="mark" href="#console_console_log_data_args"id="console_console_log_data_args">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.100</span> </div><p>Prints to <code>stdout</code> with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to <a href="http://man7.org/linux/man-pages/man3/printf.3.html">printf(3)</a> (the arguments are all passed to <a href="util.html#util_util_format_format_args"><code>util.format()</code></a>).</p> <pre><code class="lang-js">const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout </code></pre> <p>See <a href="util.html#util_util_format_format_args"><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> <div class="api_metadata"> <span>Added in: v0.1.104</span> </div><ul> <li><code>label</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> </ul> <p>Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique <code>label</code>. Use the same <code>label</code> when you call <a href="#console_console_timeend_label"><code>console.timeEnd()</code></a> to stop the timer and output the elapsed time in milliseconds to <code>stdout</code>. Timer durations are accurate to the sub-millisecond.</p> <h3>console.timeEnd(label)<span><a class="mark" href="#console_console_timeend_label"id="console_console_timeend_label">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.104</span> </div><ul> <li><code>label</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> </ul> <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 <code>stdout</code>:</p> <pre><code class="lang-js">console.time('100-elements'); for (let i = 0; i < 100; i++) ; console.timeEnd('100-elements'); // prints 100-elements: 225.438ms </code></pre> <p><em>Note: As of Node.js v6.0.0, <code>console.timeEnd()</code> deletes the timer to avoid leaking it. On older versions, the timer persisted. This allowed <code>console.timeEnd()</code> to be called multiple times for the same label. This functionality was unintended and is no longer supported.</em></p> <h3>console.trace(message[, ...args])<span><a class="mark" href="#console_console_trace_message_args"id="console_console_trace_message_args">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.104</span> </div><p>Prints to <code>stderr</code> the string <code>'Trace :'</code>, followed by the <a href="util.html#util_util_format_format_args"><code>util.format()</code></a> formatted message and stack trace to the current position in the code.</p> <pre><code class="lang-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) </code></pre> <h3>console.warn([data][, ...args])<span><a class="mark" href="#console_console_warn_data_args"id="console_console_warn_data_args">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.100</span> </div><p>The <code>console.warn()</code> function is an alias for <a href="#console_console_error_data_args"><code>console.error()</code></a>.</p> </div> </div> </div> <script src="assets/sh_main.js"></script> <script src="assets/sh_javascript.min.js"></script> <script>highlight(undefined, undefined, 'pre');</script> <!-- __TRACKING__ --> </body> </html>