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<!doctype html>
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<title>REPL | Node.js v12.0.0 Documentation</title>
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<div id="toc">
<h2>Table of Contents</h2>
<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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);</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
<span class="hljs-comment">// Entering editor mode (^D to finish, ^C to cancel)</span>
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">welcome</span>(<span class="hljs-params">name</span>) </span>{
<span class="hljs-keyword">return</span> <span class="hljs-string">`Hello <span class="hljs-subst">${name}</span>!`</span>;
}
welcome(<span class="hljs-string">'Node.js User'</span>);
<span class="hljs-comment">// ^D</span>
<span class="hljs-string">'Hello Node.js User!'</span>
></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">> <span class="hljs-number">1</span> + <span class="hljs-number">1</span>
<span class="hljs-number">2</span>
> <span class="hljs-keyword">const</span> m = <span class="hljs-number">2</span>
<span class="hljs-literal">undefined</span>
> m + <span class="hljs-number">1</span>
<span class="hljs-number">3</span></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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-keyword">const</span> msg = <span class="hljs-string">'message'</span>;
repl.start(<span class="hljs-string">'> '</span>).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
<span class="hljs-string">'message'</span></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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-keyword">const</span> msg = <span class="hljs-string">'message'</span>;
<span class="hljs-keyword">const</span> r = repl.start(<span class="hljs-string">'> '</span>);
<span class="hljs-built_in">Object</span>.defineProperty(r.context, <span class="hljs-string">'m'</span>, {
<span class="hljs-attr">configurable</span>: <span class="hljs-literal">false</span>,
<span class="hljs-attr">enumerable</span>: <span class="hljs-literal">true</span>,
<span class="hljs-attr">value</span>: 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(<span class="hljs-string">'./some/file'</span>);</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">> [ <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span> ]
[ <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span> ]
> _.length
<span class="hljs-number">3</span>
> _ += <span class="hljs-number">1</span>
Expression assignment to _ now disabled.
<span class="hljs-number">4</span>
> <span class="hljs-number">1</span> + <span class="hljs-number">1</span>
<span class="hljs-number">2</span>
> _
<span class="hljs-number">4</span></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">> <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-built_in">Error</span>(<span class="hljs-string">'foo'</span>);
<span class="hljs-built_in">Error</span>: foo
> _error.message
<span class="hljs-string">'foo'</span></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">> <span class="hljs-keyword">await</span> <span class="hljs-built_in">Promise</span>.resolve(<span class="hljs-number">123</span>)
<span class="hljs-number">123</span>
> <span class="hljs-keyword">await</span> <span class="hljs-built_in">Promise</span>.reject(<span class="hljs-keyword">new</span> <span class="hljs-built_in">Error</span>(<span class="hljs-string">'REPL await'</span>))
<span class="hljs-built_in">Error</span>: REPL <span class="hljs-keyword">await</span>
at repl:<span class="hljs-number">1</span>:<span class="hljs-number">45</span>
> <span class="hljs-keyword">const</span> timeout = util.promisify(setTimeout);
<span class="hljs-literal">undefined</span>
> <span class="hljs-keyword">const</span> old = <span class="hljs-built_in">Date</span>.now(); <span class="hljs-keyword">await</span> timeout(<span class="hljs-number">1000</span>); <span class="hljs-built_in">console</span>.log(<span class="hljs-built_in">Date</span>.now() - old);
<span class="hljs-number">1002</span>
<span class="hljs-literal">undefined</span></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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-keyword">const</span> { Translator } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'translator'</span>);
<span class="hljs-keyword">const</span> myTranslator = <span class="hljs-keyword">new</span> Translator(<span class="hljs-string">'en'</span>, <span class="hljs-string">'fr'</span>);
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">myEval</span>(<span class="hljs-params">cmd, context, filename, callback</span>) </span>{
callback(<span class="hljs-literal">null</span>, myTranslator.translate(cmd));
}
repl.start({ <span class="hljs-attr">prompt</span>: <span class="hljs-string">'> '</span>, <span class="hljs-attr">eval</span>: 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"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">myEval</span>(<span class="hljs-params">cmd, context, filename, callback</span>) </span>{
<span class="hljs-keyword">let</span> result;
<span class="hljs-keyword">try</span> {
result = vm.runInThisContext(cmd);
} <span class="hljs-keyword">catch</span> (e) {
<span class="hljs-keyword">if</span> (isRecoverableError(e)) {
<span class="hljs-keyword">return</span> callback(<span class="hljs-keyword">new</span> repl.Recoverable(e));
}
}
callback(<span class="hljs-literal">null</span>, result);
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">isRecoverableError</span>(<span class="hljs-params">error</span>) </span>{
<span class="hljs-keyword">if</span> (error.name === <span class="hljs-string">'SyntaxError'</span>) {
<span class="hljs-keyword">return</span> <span class="hljs-regexp">/^(Unexpected end of input|Unexpected token)/</span>.test(error.message);
}
<span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>;
}</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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-keyword">const</span> r = repl.start({ <span class="hljs-attr">prompt</span>: <span class="hljs-string">'> '</span>, <span class="hljs-attr">eval</span>: myEval, <span class="hljs-attr">writer</span>: myWriter });
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">myEval</span>(<span class="hljs-params">cmd, context, filename, callback</span>) </span>{
callback(<span class="hljs-literal">null</span>, cmd);
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">myWriter</span>(<span class="hljs-params">output</span>) </span>{
<span class="hljs-keyword">return</span> 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(<span class="hljs-string">'exit'</span>, () => {
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">'Received "exit" event from repl!'</span>);
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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">initializeContext</span>(<span class="hljs-params">context</span>) </span>{
context.m = <span class="hljs-string">'test'</span>;
}
<span class="hljs-keyword">const</span> r = repl.start({ <span class="hljs-attr">prompt</span>: <span class="hljs-string">'> '</span> });
initializeContext(r.context);
r.on(<span class="hljs-string">'reset'</span>, 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
<span class="hljs-string">'test'</span>
> m = <span class="hljs-number">1</span>
<span class="hljs-number">1</span>
> m
<span class="hljs-number">1</span>
> .clear
Clearing context...
> m
<span class="hljs-string">'test'</span>
></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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-keyword">const</span> replServer = repl.start({ <span class="hljs-attr">prompt</span>: <span class="hljs-string">'> '</span> });
replServer.defineCommand(<span class="hljs-string">'sayhello'</span>, {
<span class="hljs-attr">help</span>: <span class="hljs-string">'Say hello'</span>,
action(name) {
<span class="hljs-keyword">this</span>.clearBufferedCommand();
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">`Hello, <span class="hljs-subst">${name}</span>!`</span>);
<span class="hljs-keyword">this</span>.displayPrompt();
}
});
replServer.defineCommand(<span class="hljs-string">'saybye'</span>, <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">saybye</span>(<span class="hljs-params"></span>) </span>{
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">'Goodbye!'</span>);
<span class="hljs-keyword">this</span>.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/runkitdev/node/blob/cf1ce2bcc1a88ef6ff1bf745a19913a69607fb02/lib/repl.js#L757">[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"><span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-comment">// a Unix style prompt</span>
repl.start(<span class="hljs-string">'$ '</span>);</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
> <span class="hljs-keyword">const</span> a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>];
<span class="hljs-literal">undefined</span>
> a
[ <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span> ]
> a.forEach(<span class="hljs-function">(<span class="hljs-params">v</span>) =></span> {
... console.log(v);
... });
<span class="hljs-number">1</span>
<span class="hljs-number">2</span>
<span class="hljs-number">3</span></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"><span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'net'</span>);
<span class="hljs-keyword">const</span> repl = <span class="hljs-built_in">require</span>(<span class="hljs-string">'repl'</span>);
<span class="hljs-keyword">let</span> connections = <span class="hljs-number">0</span>;
repl.start({
<span class="hljs-attr">prompt</span>: <span class="hljs-string">'Node.js via stdin> '</span>,
<span class="hljs-attr">input</span>: process.stdin,
<span class="hljs-attr">output</span>: process.stdout
});
net.createServer(<span class="hljs-function">(<span class="hljs-params">socket</span>) =></span> {
connections += <span class="hljs-number">1</span>;
repl.start({
<span class="hljs-attr">prompt</span>: <span class="hljs-string">'Node.js via Unix socket> '</span>,
<span class="hljs-attr">input</span>: socket,
<span class="hljs-attr">output</span>: socket
}).on(<span class="hljs-string">'exit'</span>, () => {
socket.end();
});
}).listen(<span class="hljs-string">'/tmp/node-repl-sock'</span>);
net.createServer(<span class="hljs-function">(<span class="hljs-params">socket</span>) =></span> {
connections += <span class="hljs-number">1</span>;
repl.start({
<span class="hljs-attr">prompt</span>: <span class="hljs-string">'Node.js via TCP socket> '</span>,
<span class="hljs-attr">input</span>: socket,
<span class="hljs-attr">output</span>: socket
}).on(<span class="hljs-string">'exit'</span>, () => {
socket.end();
});
}).listen(<span class="hljs-number">5001</span>);</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>
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