<!doctype html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width"> <title>REPL | Node.js v10.15.3 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/repl.html"> </head> <body class="alt apidoc" id="api-section-repl"> <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 href="documentation.html" class="nav-documentation">About these Docs</a></li> <li><a href="synopsis.html" class="nav-synopsis">Usage & Example</a></li> </ul> <div class="line"></div> <ul> <li><a href="assert.html" class="nav-assert">Assertion Testing</a></li> <li><a href="async_hooks.html" class="nav-async_hooks">Async Hooks</a></li> <li><a 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<ul> <li> <p><span class="stability_2"><a href="#repl_repl">REPL</a></span></p> <ul> <li> <p><a href="#repl_design_and_features">Design and Features</a></p> <ul> <li><a href="#repl_commands_and_special_keys">Commands and Special Keys</a></li> <li> <p><a href="#repl_default_evaluation">Default Evaluation</a></p> <ul> <li><a href="#repl_javascript_expressions">JavaScript Expressions</a></li> <li><a href="#repl_global_and_local_scope">Global and Local Scope</a></li> <li><a href="#repl_accessing_core_node_js_modules">Accessing Core Node.js Modules</a></li> <li><a href="#repl_global_uncaught_exceptions">Global Uncaught Exceptions</a></li> <li><a href="#repl_assignment_of_the_underscore_variable">Assignment of the <code>_</code> (underscore) variable</a></li> <li><a href="#repl_await_keyword"><code>await</code> keyword</a></li> </ul> </li> <li> <p><a href="#repl_custom_evaluation_functions">Custom Evaluation Functions</a></p> <ul> <li><a href="#repl_recoverable_errors">Recoverable Errors</a></li> </ul> </li> <li><a href="#repl_customizing_repl_output">Customizing REPL Output</a></li> </ul> </li> <li> <p><a href="#repl_class_replserver">Class: REPLServer</a></p> <ul> <li><a href="#repl_event_exit">Event: 'exit'</a></li> <li><a href="#repl_event_reset">Event: 'reset'</a></li> <li><a href="#repl_replserver_definecommand_keyword_cmd">replServer.defineCommand(keyword, cmd)</a></li> <li><a href="#repl_replserver_displayprompt_preservecursor">replServer.displayPrompt([preserveCursor])</a></li> <li><a href="#repl_replserver_clearbufferedcommand">replServer.clearBufferedCommand()</a></li> <li><span class="stability_0"><a href="#repl_replserver_parsereplkeyword_keyword_rest">replServer.parseREPLKeyword(keyword[, rest])</a></span></li> </ul> </li> <li><a href="#repl_repl_start_options">repl.start([options])</a></li> <li> <p><a href="#repl_the_node_js_repl">The Node.js REPL</a></p> <ul> <li><a href="#repl_environment_variable_options">Environment Variable Options</a></li> <li><a href="#repl_persistent_history">Persistent History</a></li> <li><a href="#repl_using_the_node_js_repl_with_advanced_line_editors">Using the Node.js REPL with advanced line-editors</a></li> <li><a href="#repl_starting_multiple_repl_instances_against_a_single_running_instance">Starting multiple REPL instances against a single running instance</a></li> </ul> </li> </ul> </li> </ul> </div> <div id="apicontent"> <h1>REPL<span><a class="mark" href="#repl_repl" id="repl_repl">#</a></span></h1> <p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p> <p>The <code>repl</code> module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:</p> <pre><code class="language-js">const repl = require('repl'); </code></pre> <h2>Design and Features<span><a class="mark" href="#repl_design_and_features" id="repl_design_and_features">#</a></span></h2> <p>The <code>repl</code> module exports the <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> class. While running, instances of <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from <code>stdin</code> and <code>stdout</code>, respectively, or may be connected to any Node.js <a href="stream.html">stream</a>.</p> <p>Instances of <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> support automatic completion of inputs, simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions.</p> <h3>Commands and Special Keys<span><a class="mark" href="#repl_commands_and_special_keys" id="repl_commands_and_special_keys">#</a></span></h3> <p>The following special commands are supported by all REPL instances:</p> <ul> <li><code>.break</code> - When in the process of inputting a multi-line expression, entering the <code>.break</code> command (or pressing the <code><ctrl>-C</code> key combination) will abort further input or processing of that expression.</li> <li><code>.clear</code> - Resets the REPL <code>context</code> to an empty object and clears any multi-line expression currently being input.</li> <li><code>.exit</code> - Close the I/O stream, causing the REPL to exit.</li> <li><code>.help</code> - Show this list of special commands.</li> <li><code>.save</code> - Save the current REPL session to a file: <code>> .save ./file/to/save.js</code></li> <li><code>.load</code> - Load a file into the current REPL session. <code>> .load ./file/to/load.js</code></li> <li><code>.editor</code> - Enter editor mode (<code><ctrl>-D</code> to finish, <code><ctrl>-C</code> to cancel).</li> </ul> <!-- eslint-skip --> <pre><code class="language-js">> .editor // Entering editor mode (^D to finish, ^C to cancel) function welcome(name) { return `Hello ${name}!`; } welcome('Node.js User'); // ^D 'Hello Node.js User!' > </code></pre> <p>The following key combinations in the REPL have these special effects:</p> <ul> <li><code><ctrl>-C</code> - When pressed once, has the same effect as the <code>.break</code> command. When pressed twice on a blank line, has the same effect as the <code>.exit</code> command.</li> <li><code><ctrl>-D</code> - Has the same effect as the <code>.exit</code> command.</li> <li><code><tab></code> - When pressed on a blank line, displays global and local (scope) variables. When pressed while entering other input, displays relevant autocompletion options.</li> </ul> <h3>Default Evaluation<span><a class="mark" href="#repl_default_evaluation" id="repl_default_evaluation">#</a></span></h3> <p>By default, all instances of <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> use an evaluation function that evaluates JavaScript expressions and provides access to Node.js' built-in modules. This default behavior can be overridden by passing in an alternative evaluation function when the <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> instance is created.</p> <h4>JavaScript Expressions<span><a class="mark" href="#repl_javascript_expressions" id="repl_javascript_expressions">#</a></span></h4> <p>The default evaluator supports direct evaluation of JavaScript expressions:</p> <!-- eslint-skip --> <pre><code class="language-js">> 1 + 1 2 > const m = 2 undefined > m + 1 3 </code></pre> <p>Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the <code>const</code>, <code>let</code>, or <code>var</code> keywords are declared at the global scope.</p> <h4>Global and Local Scope<span><a class="mark" href="#repl_global_and_local_scope" id="repl_global_and_local_scope">#</a></span></h4> <p>The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the <code>context</code> object associated with each <code>REPLServer</code>:</p> <pre><code class="language-js">const repl = require('repl'); const msg = 'message'; repl.start('> ').context.m = msg; </code></pre> <p>Properties in the <code>context</code> object appear as local within the REPL:</p> <!-- eslint-skip --> <pre><code class="language-js">$ node repl_test.js > m 'message' </code></pre> <p>Context properties are not read-only by default. To specify read-only globals, context properties must be defined using <code>Object.defineProperty()</code>:</p> <pre><code class="language-js">const repl = require('repl'); const msg = 'message'; const r = repl.start('> '); Object.defineProperty(r.context, 'm', { configurable: false, enumerable: true, value: msg }); </code></pre> <h4>Accessing Core Node.js Modules<span><a class="mark" href="#repl_accessing_core_node_js_modules" id="repl_accessing_core_node_js_modules">#</a></span></h4> <p>The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input <code>fs</code> will be evaluated on-demand as <code>global.fs = require('fs')</code>.</p> <!-- eslint-skip --> <pre><code class="language-js">> fs.createReadStream('./some/file'); </code></pre> <h4>Global Uncaught Exceptions<span><a class="mark" href="#repl_global_uncaught_exceptions" id="repl_global_uncaught_exceptions">#</a></span></h4> <p>The REPL uses the <a href="domain.html"><code>domain</code></a> module to catch all uncaught exceptions for that REPL session.</p> <p>This use of the <a href="domain.html"><code>domain</code></a> module in the REPL has these side effects:</p> <ul> <li>Uncaught exceptions do not emit the <a href="process.html#process_event_uncaughtexception"><code>'uncaughtException'</code></a> event.</li> <li>Trying to use <a href="process.html#process_process_setuncaughtexceptioncapturecallback_fn"><code>process.setUncaughtExceptionCaptureCallback()</code></a> throws an <a href="errors.html#errors_err_domain_cannot_set_uncaught_exception_capture"><code>ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE</code></a> error.</li> </ul> <h4>Assignment of the `_` (underscore) variable<span><a class="mark" href="#repl_assignment_of_the_underscore_variable" id="repl_assignment_of_the_underscore_variable">#</a></span></h4> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v9.8.0</td> <td><p>Added <code>_error</code> support.</p></td></tr> </tbody></table> </details> </div> <p>The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable <code>_</code> (underscore). Explicitly setting <code>_</code> to a value will disable this behavior.</p> <!-- eslint-skip --> <pre><code class="language-js">> [ 'a', 'b', 'c' ] [ 'a', 'b', 'c' ] > _.length 3 > _ += 1 Expression assignment to _ now disabled. 4 > 1 + 1 2 > _ 4 </code></pre> <p>Similarly, <code>_error</code> will refer to the last seen error, if there was any. Explicitly setting <code>_error</code> to a value will disable this behavior.</p> <!-- eslint-skip --> <pre><code class="language-js">> throw new Error('foo'); Error: foo > _error.message 'foo' </code></pre> <h4>`await` keyword<span><a class="mark" href="#repl_await_keyword" id="repl_await_keyword">#</a></span></h4> <p>With the <a href="cli.html#cli_experimental_repl_await"><code>--experimental-repl-await</code></a> command line option specified, experimental support for the <code>await</code> keyword is enabled.</p> <!-- eslint-skip --> <pre><code class="language-js">> await Promise.resolve(123) 123 > await Promise.reject(new Error('REPL await')) Error: REPL await at repl:1:45 > const timeout = util.promisify(setTimeout); undefined > const old = Date.now(); await timeout(1000); console.log(Date.now() - old); 1002 undefined </code></pre> <h3>Custom Evaluation Functions<span><a class="mark" href="#repl_custom_evaluation_functions" id="repl_custom_evaluation_functions">#</a></span></h3> <p>When a new <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> is created, a custom evaluation function may be provided. This can be used, for instance, to implement fully customized REPL applications.</p> <p>The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:</p> <pre><code class="language-js">const repl = require('repl'); const { Translator } = require('translator'); const myTranslator = new Translator('en', 'fr'); function myEval(cmd, context, filename, callback) { callback(null, myTranslator.translate(cmd)); } repl.start({ prompt: '> ', eval: myEval }); </code></pre> <h4>Recoverable Errors<span><a class="mark" href="#repl_recoverable_errors" id="repl_recoverable_errors">#</a></span></h4> <p>As a user is typing input into the REPL prompt, pressing the <code><enter></code> key will send the current line of input to the <code>eval</code> function. In order to support multi-line input, the eval function can return an instance of <code>repl.Recoverable</code> to the provided callback function:</p> <pre><code class="language-js">function myEval(cmd, context, filename, callback) { let result; try { result = vm.runInThisContext(cmd); } catch (e) { if (isRecoverableError(e)) { return callback(new repl.Recoverable(e)); } } callback(null, result); } function isRecoverableError(error) { if (error.name === 'SyntaxError') { return /^(Unexpected end of input|Unexpected token)/.test(error.message); } return false; } </code></pre> <h3>Customizing REPL Output<span><a class="mark" href="#repl_customizing_repl_output" id="repl_customizing_repl_output">#</a></span></h3> <p>By default, <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> instances format output using the <a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> method before writing the output to the provided <code>Writable</code> stream (<code>process.stdout</code> by default). The <code>useColors</code> boolean option can be specified at construction to instruct the default writer to use ANSI style codes to colorize the output from the <code>util.inspect()</code> method.</p> <p>It is possible to fully customize the output of a <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> instance by passing a new function in using the <code>writer</code> option on construction. The following example, for instance, simply converts any input text to upper case:</p> <pre><code class="language-js">const repl = require('repl'); const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter }); function myEval(cmd, context, filename, callback) { callback(null, cmd); } function myWriter(output) { return output.toUpperCase(); } </code></pre> <h2>Class: REPLServer<span><a class="mark" href="#repl_class_replserver" id="repl_class_replserver">#</a></span></h2> <div class="api_metadata"> <span>Added in: v0.1.91</span> </div> <p>The <code>repl.REPLServer</code> class inherits from the <a href="readline.html#readline_class_interface"><code>readline.Interface</code></a> class. Instances of <code>repl.REPLServer</code> are created using the <code>repl.start()</code> method and <em>should not</em> be created directly using the JavaScript <code>new</code> keyword.</p> <h3>Event: 'exit'<span><a class="mark" href="#repl_event_exit" id="repl_event_exit">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.7.7</span> </div> <p>The <code>'exit'</code> event is emitted when the REPL is exited either by receiving the <code>.exit</code> command as input, the user pressing <code><ctrl>-C</code> twice to signal <code>SIGINT</code>, or by pressing <code><ctrl>-D</code> to signal <code>'end'</code> on the input stream. The listener callback is invoked without any arguments.</p> <pre><code class="language-js">replServer.on('exit', () => { console.log('Received "exit" event from repl!'); process.exit(); }); </code></pre> <h3>Event: 'reset'<span><a class="mark" href="#repl_event_reset" id="repl_event_reset">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.11.0</span> </div> <p>The <code>'reset'</code> event is emitted when the REPL's context is reset. This occurs whenever the <code>.clear</code> command is received as input <em>unless</em> the REPL is using the default evaluator and the <code>repl.REPLServer</code> instance was created with the <code>useGlobal</code> option set to <code>true</code>. The listener callback will be called with a reference to the <code>context</code> object as the only argument.</p> <p>This can be used primarily to re-initialize REPL context to some pre-defined state:</p> <pre><code class="language-js">const repl = require('repl'); function initializeContext(context) { context.m = 'test'; } const r = repl.start({ prompt: '> ' }); initializeContext(r.context); r.on('reset', initializeContext); </code></pre> <p>When this code is executed, the global <code>'m'</code> variable can be modified but then reset to its initial value using the <code>.clear</code> command:</p> <!-- eslint-skip --> <pre><code class="language-js">$ ./node example.js > m 'test' > m = 1 1 > m 1 > .clear Clearing context... > m 'test' > </code></pre> <h3>replServer.defineCommand(keyword, cmd)<span><a class="mark" href="#repl_replserver_definecommand_keyword_cmd" id="repl_replserver_definecommand_keyword_cmd">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.3.0</span> </div> <ul> <li><code>keyword</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The command keyword (<em>without</em> a leading <code>.</code> character).</li> <li><code>cmd</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to invoke when the command is processed.</li> </ul> <p>The <code>replServer.defineCommand()</code> method is used to add new <code>.</code>-prefixed commands to the REPL instance. Such commands are invoked by typing a <code>.</code> followed by the <code>keyword</code>. The <code>cmd</code> is either a <code>Function</code> or an <code>Object</code> with the following properties:</p> <ul> <li><code>help</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> Help text to be displayed when <code>.help</code> is entered (Optional).</li> <li><code>action</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to execute, optionally accepting a single string argument.</li> </ul> <p>The following example shows two new commands added to the REPL instance:</p> <pre><code class="language-js">const repl = require('repl'); const replServer = repl.start({ prompt: '> ' }); replServer.defineCommand('sayhello', { help: 'Say hello', action(name) { this.clearBufferedCommand(); console.log(`Hello, ${name}!`); this.displayPrompt(); } }); replServer.defineCommand('saybye', function saybye() { console.log('Goodbye!'); this.close(); }); </code></pre> <p>The new commands can then be used from within the REPL instance:</p> <pre><code class="language-txt">> .sayhello Node.js User Hello, Node.js User! > .saybye Goodbye! </code></pre> <h3>replServer.displayPrompt([preserveCursor])<span><a class="mark" href="#repl_replserver_displayprompt_preservecursor" id="repl_replserver_displayprompt_preservecursor">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.1.91</span> </div> <ul> <li><code>preserveCursor</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> </ul> <p>The <code>replServer.displayPrompt()</code> method readies the REPL instance for input from the user, printing the configured <code>prompt</code> to a new line in the <code>output</code> and resuming the <code>input</code> to accept new input.</p> <p>When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.</p> <p>When <code>preserveCursor</code> is <code>true</code>, the cursor placement will not be reset to <code>0</code>.</p> <p>The <code>replServer.displayPrompt</code> method is primarily intended to be called from within the action function for commands registered using the <code>replServer.defineCommand()</code> method.</p> <h3>replServer.clearBufferedCommand()<span><a class="mark" href="#repl_replserver_clearbufferedcommand" id="repl_replserver_clearbufferedcommand">#</a></span></h3> <div class="api_metadata"> <span>Added in: v9.0.0</span> </div> <p>The <code>replServer.clearBufferedCommand()</code> method clears any command that has been buffered but not yet executed. This method is primarily intended to be called from within the action function for commands registered using the <code>replServer.defineCommand()</code> method.</p> <h3>replServer.parseREPLKeyword(keyword[, rest])<span><a class="mark" href="#repl_replserver_parsereplkeyword_keyword_rest" id="repl_replserver_parsereplkeyword_keyword_rest">#</a></span></h3> <div class="api_metadata"> <span>Added in: v0.8.9</span><span>Deprecated since: v9.0.0</span> </div> <ul> <li><code>keyword</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> the potential keyword to parse and execute</li> <li><code>rest</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> any parameters to the keyword command</li> <li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> </ul> <p></p><div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated.</div><p></p> <p>An internal method used to parse and execute <code>REPLServer</code> keywords. Returns <code>true</code> if <code>keyword</code> is a valid keyword, otherwise <code>false</code>.</p> <h2>repl.start([options])<a class="srclink" href="https://github.com/nodejs/node/blob/1a96d83a223ff9f05f7d942fb84440d323f7b596/lib/repl.js#L756">[src]</a><span><a class="mark" href="#repl_repl_start_options" id="repl_repl_start_options">#</a></span></h2> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v10.0.0</td> <td><p>The <code>REPL_MAGIC_MODE</code> <code>replMode</code> was removed.</p></td></tr> <tr><td>v5.8.0</td> <td><p>The <code>options</code> parameter is optional now.</p></td></tr> <tr><td>v0.1.91</td> <td><p><span>Added in: v0.1.91</span></p></td></tr> </tbody></table> </details> </div> <ul> <li> <p><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></p> <ul> <li><code>prompt</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The input prompt to display. <strong>Default:</strong> <code>'> '</code> (with a trailing space).</li> <li><code>input</code> <a href="stream.html#stream_class_stream_readable" class="type"><stream.Readable></a> The <code>Readable</code> stream from which REPL input will be read. <strong>Default:</strong> <code>process.stdin</code>.</li> <li><code>output</code> <a href="stream.html#stream_class_stream_writable" class="type"><stream.Writable></a> The <code>Writable</code> stream to which REPL output will be written. <strong>Default:</strong> <code>process.stdout</code>.</li> <li><code>terminal</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If <code>true</code>, specifies that the <code>output</code> should be treated as a TTY terminal, and have ANSI/VT100 escape codes written to it. <strong>Default:</strong> checking the value of the <code>isTTY</code> property on the <code>output</code> stream upon instantiation.</li> <li><code>eval</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to be used when evaluating each given line of input. <strong>Default:</strong> an async wrapper for the JavaScript <code>eval()</code> function. An <code>eval</code> function can error with <code>repl.Recoverable</code> to indicate the input was incomplete and prompt for additional lines.</li> <li><code>useColors</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If <code>true</code>, specifies that the default <code>writer</code> function should include ANSI color styling to REPL output. If a custom <code>writer</code> function is provided then this has no effect. <strong>Default:</strong> the REPL instances <code>terminal</code> value.</li> <li><code>useGlobal</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If <code>true</code>, specifies that the default evaluation function will use the JavaScript <code>global</code> as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value to <code>true</code>. <strong>Default:</strong> <code>false</code>.</li> <li><code>ignoreUndefined</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If <code>true</code>, specifies that the default writer will not output the return value of a command if it evaluates to <code>undefined</code>. <strong>Default:</strong> <code>false</code>.</li> <li><code>writer</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to invoke to format the output of each command before writing to <code>output</code>. <strong>Default:</strong> <a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a>.</li> <li><code>completer</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> An optional function used for custom Tab auto completion. See <a href="readline.html#readline_use_of_the_completer_function"><code>readline.InterfaceCompleter</code></a> for an example.</li> <li> <p><code>replMode</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:</p> <ul> <li><code>repl.REPL_MODE_SLOPPY</code> - evaluates expressions in sloppy mode.</li> <li><code>repl.REPL_MODE_STRICT</code> - evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with <code>'use strict'</code>.</li> </ul> </li> <li><code>breakEvalOnSigint</code> - Stop evaluating the current piece of code when <code>SIGINT</code> is received, i.e. <code>Ctrl+C</code> is pressed. This cannot be used together with a custom <code>eval</code> function. <strong>Default:</strong> <code>false</code>.</li> </ul> </li> <li>Returns: <a href="repl.html#repl_class_replserver" class="type"><repl.REPLServer></a></li> </ul> <p>The <code>repl.start()</code> method creates and starts a <a href="#repl_class_replserver"><code>repl.REPLServer</code></a> instance.</p> <p>If <code>options</code> is a string, then it specifies the input prompt:</p> <pre><code class="language-js">const repl = require('repl'); // a Unix style prompt repl.start('$ '); </code></pre> <h2>The Node.js REPL<span><a class="mark" href="#repl_the_node_js_repl" id="repl_the_node_js_repl">#</a></span></h2> <p>Node.js itself uses the <code>repl</code> module to provide its own interactive interface for executing JavaScript. This can be used by executing the Node.js binary without passing any arguments (or by passing the <code>-i</code> argument):</p> <!-- eslint-skip --> <pre><code class="language-js">$ node > const a = [1, 2, 3]; undefined > a [ 1, 2, 3 ] > a.forEach((v) => { ... console.log(v); ... }); 1 2 3 </code></pre> <h3>Environment Variable Options<span><a class="mark" href="#repl_environment_variable_options" id="repl_environment_variable_options">#</a></span></h3> <p>Various behaviors of the Node.js REPL can be customized using the following environment variables:</p> <ul> <li><code>NODE_REPL_HISTORY</code> - When a valid path is given, persistent REPL history will be saved to the specified file rather than <code>.node_repl_history</code> in the user's home directory. Setting this value to <code>''</code> will disable persistent REPL history. Whitespace will be trimmed from the value.</li> <li><code>NODE_REPL_HISTORY_SIZE</code> - Controls how many lines of history will be persisted if history is available. Must be a positive number. <strong>Default:</strong> <code>1000</code>.</li> <li><code>NODE_REPL_MODE</code> - May be either <code>'sloppy'</code> or <code>'strict'</code>. <strong>Default:</strong> <code>'sloppy'</code>, which will allow non-strict mode code to be run.</li> </ul> <h3>Persistent History<span><a class="mark" href="#repl_persistent_history" id="repl_persistent_history">#</a></span></h3> <p>By default, the Node.js REPL will persist history between <code>node</code> REPL sessions by saving inputs to a <code>.node_repl_history</code> file located in the user's home directory. This can be disabled by setting the environment variable <code>NODE_REPL_HISTORY=''</code>.</p> <h3>Using the Node.js REPL with advanced line-editors<span><a class="mark" href="#repl_using_the_node_js_repl_with_advanced_line_editors" id="repl_using_the_node_js_repl_with_advanced_line_editors">#</a></span></h3> <p>For advanced line-editors, start Node.js with the environment variable <code>NODE_NO_READLINE=1</code>. This will start the main and debugger REPL in canonical terminal settings, which will allow use with <code>rlwrap</code>.</p> <p>For example, the following can be added to a <code>.bashrc</code> file:</p> <pre><code class="language-text">alias node="env NODE_NO_READLINE=1 rlwrap node" </code></pre> <h3>Starting multiple REPL instances against a single running instance<span><a class="mark" href="#repl_starting_multiple_repl_instances_against_a_single_running_instance" id="repl_starting_multiple_repl_instances_against_a_single_running_instance">#</a></span></h3> <p>It is possible to create and run multiple REPL instances against a single running instance of Node.js that share a single <code>global</code> object but have separate I/O interfaces.</p> <p>The following example, for instance, provides separate REPLs on <code>stdin</code>, a Unix socket, and a TCP socket:</p> <pre><code class="language-js">const net = require('net'); const repl = require('repl'); let connections = 0; repl.start({ prompt: 'Node.js via stdin> ', input: process.stdin, output: process.stdout }); net.createServer((socket) => { connections += 1; repl.start({ prompt: 'Node.js via Unix socket> ', input: socket, output: socket }).on('exit', () => { socket.end(); }); }).listen('/tmp/node-repl-sock'); net.createServer((socket) => { connections += 1; repl.start({ prompt: 'Node.js via TCP socket> ', input: socket, output: socket }).on('exit', () => { socket.end(); }); }).listen(5001); </code></pre> <p>Running this application from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. <code>telnet</code>, for instance, is useful for connecting to TCP sockets, while <code>socat</code> can be used to connect to both Unix and TCP sockets.</p> <p>By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.</p> <p>For an example of running a "full-featured" (<code>terminal</code>) REPL over a <code>net.Server</code> and <code>net.Socket</code> instance, see: <a href="https://gist.github.com/TooTallNate/2209310">https://gist.github.com/TooTallNate/2209310</a>.</p> <p>For an example of running a REPL instance over <a href="https://curl.haxx.se/docs/manpage.html"></a><a href="http://man7.org/linux/man-pages/man1/curl.1.html"><code>curl(1)</code></a>, see: <a href="https://gist.github.com/TooTallNate/2053342">https://gist.github.com/TooTallNate/2053342</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> </body> </html>