The repl module provides a Read-Eval-Print-Loop (REPL) implementation that\nis available both as a standalone program or includible in other applications.\nIt can be accessed using:
const repl = require('repl');\nThe repl module exports the repl.REPLServer class. While running,\ninstances of repl.REPLServer will accept individual lines of user input,\nevaluate those according to a user-defined evaluation function, then output the\nresult. Input and output may be from stdin and stdout, respectively, or may\nbe connected to any Node.js stream.
Instances of repl.REPLServer support automatic completion of inputs,\nsimplistic Emacs-style line editing, multi-line inputs, ANSI-styled output,\nsaving and restoring current REPL session state, error recovery, and\ncustomizable evaluation functions.
The following special commands are supported by all REPL instances:
\n.break - When in the process of inputting a multi-line expression, entering\nthe .break command (or pressing the <ctrl>-C key combination) will abort\nfurther input or processing of that expression..clear - Resets the REPL context to an empty object and clears any\nmulti-line expression currently being input..exit - Close the I/O stream, causing the REPL to exit..help - Show this list of special commands..save - Save the current REPL session to a file:\n> .save ./file/to/save.js.load - Load a file into the current REPL session.\n> .load ./file/to/load.js.editor - Enter editor mode (<ctrl>-D to finish, <ctrl>-C to cancel).> .editor\n// Entering editor mode (^D to finish, ^C to cancel)\nfunction welcome(name) {\n return `Hello ${name}!`;\n}\n\nwelcome('Node.js User');\n\n// ^D\n'Hello Node.js User!'\n>\nThe following key combinations in the REPL have these special effects:
\n<ctrl>-C - When pressed once, has the same effect as the .break command.\nWhen pressed twice on a blank line, has the same effect as the .exit\ncommand.<ctrl>-D - Has the same effect as the .exit command.<tab> - When pressed on a blank line, displays global and local (scope)\nvariables. When pressed while entering other input, displays relevant\nautocompletion options.By default, all instances of repl.REPLServer use an evaluation function\nthat evaluates JavaScript expressions and provides access to Node.js' built-in\nmodules. This default behavior can be overridden by passing in an alternative\nevaluation function when the repl.REPLServer instance is created.
The default evaluator supports direct evaluation of JavaScript expressions:
\n\n> 1 + 1\n2\n> const m = 2\nundefined\n> m + 1\n3\nUnless otherwise scoped within blocks or functions, variables declared\neither implicitly or using the const, let, or var keywords\nare declared at the global scope.
The default evaluator provides access to any variables that exist in the global\nscope. It is possible to expose a variable to the REPL explicitly by assigning\nit to the context object associated with each REPLServer:
const repl = require('repl');\nconst msg = 'message';\n\nrepl.start('> ').context.m = msg;\nProperties in the context object appear as local within the REPL:
$ node repl_test.js\n> m\n'message'\nContext properties are not read-only by default. To specify read-only globals,\ncontext properties must be defined using Object.defineProperty():
const repl = require('repl');\nconst msg = 'message';\n\nconst r = repl.start('> ');\nObject.defineProperty(r.context, 'm', {\n configurable: false,\n enumerable: true,\n value: msg\n});\nThe default evaluator will automatically load Node.js core modules into the\nREPL environment when used. For instance, unless otherwise declared as a\nglobal or scoped variable, the input fs will be evaluated on-demand as\nglobal.fs = require('fs').
> fs.createReadStream('./some/file');\nThe REPL uses the domain module to catch all uncaught exceptions for that\nREPL session.
This use of the domain module in the REPL has these side effects:
'uncaughtException' event.process.setUncaughtExceptionCaptureCallback() throws\nan ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE error.The default evaluator will, by default, assign the result of the most recently\nevaluated expression to the special variable _ (underscore).\nExplicitly setting _ to a value will disable this behavior.
> [ 'a', 'b', 'c' ]\n[ 'a', 'b', 'c' ]\n> _.length\n3\n> _ += 1\nExpression assignment to _ now disabled.\n4\n> 1 + 1\n2\n> _\n4\nSimilarly, _error will refer to the last seen error, if there was any.\nExplicitly setting _error to a value will disable this behavior.
> throw new Error('foo');\nError: foo\n> _error.message\n'foo'\nWith the --experimental-repl-await command line option specified,\nexperimental support for the await keyword is enabled.
> await Promise.resolve(123)\n123\n> await Promise.reject(new Error('REPL await'))\nError: REPL await\n at repl:1:45\n> const timeout = util.promisify(setTimeout);\nundefined\n> const old = Date.now(); await timeout(1000); console.log(Date.now() - old);\n1002\nundefined\nWhen a new repl.REPLServer is created, a custom evaluation function may be\nprovided. This can be used, for instance, to implement fully customized REPL\napplications.
The following illustrates a hypothetical example of a REPL that performs\ntranslation of text from one language to another:
\nconst repl = require('repl');\nconst { Translator } = require('translator');\n\nconst myTranslator = new Translator('en', 'fr');\n\nfunction myEval(cmd, context, filename, callback) {\n callback(null, myTranslator.translate(cmd));\n}\n\nrepl.start({ prompt: '> ', eval: myEval });\nAs a user is typing input into the REPL prompt, pressing the <enter> key will\nsend the current line of input to the eval function. In order to support\nmulti-line input, the eval function can return an instance of repl.Recoverable\nto the provided callback function:
function myEval(cmd, context, filename, callback) {\n let result;\n try {\n result = vm.runInThisContext(cmd);\n } catch (e) {\n if (isRecoverableError(e)) {\n return callback(new repl.Recoverable(e));\n }\n }\n callback(null, result);\n}\n\nfunction isRecoverableError(error) {\n if (error.name === 'SyntaxError') {\n return /^(Unexpected end of input|Unexpected token)/.test(error.message);\n }\n return false;\n}\nBy default, repl.REPLServer instances format output using the\nutil.inspect() method before writing the output to the provided Writable\nstream (process.stdout by default). The useColors boolean option can be\nspecified at construction to instruct the default writer to use ANSI style\ncodes to colorize the output from the util.inspect() method.
It is possible to fully customize the output of a repl.REPLServer instance\nby passing a new function in using the writer option on construction. The\nfollowing example, for instance, simply converts any input text to upper case:
const repl = require('repl');\n\nconst r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });\n\nfunction myEval(cmd, context, filename, callback) {\n callback(null, cmd);\n}\n\nfunction myWriter(output) {\n return output.toUpperCase();\n}\nNode.js itself uses the repl module to provide its own interactive interface\nfor executing JavaScript. This can be used by executing the Node.js binary\nwithout passing any arguments (or by passing the -i argument):
$ node\n> const a = [1, 2, 3];\nundefined\n> a\n[ 1, 2, 3 ]\n> a.forEach((v) => {\n... console.log(v);\n... });\n1\n2\n3\nVarious behaviors of the Node.js REPL can be customized using the following\nenvironment variables:
\nNODE_REPL_HISTORY - When a valid path is given, persistent REPL history\nwill be saved to the specified file rather than .node_repl_history in the\nuser's home directory. Setting this value to '' will disable persistent\nREPL history. Whitespace will be trimmed from the value.NODE_REPL_HISTORY_SIZE - Controls how many lines of history will be\npersisted if history is available. Must be a positive number.\nDefault: 1000.NODE_REPL_MODE - May be either 'sloppy' or 'strict'. Default:\n'sloppy', which will allow non-strict mode code to be run.By default, the Node.js REPL will persist history between node REPL sessions\nby saving inputs to a .node_repl_history file located in the user's home\ndirectory. This can be disabled by setting the environment variable\nNODE_REPL_HISTORY=''.
For advanced line-editors, start Node.js with the environment variable\nNODE_NO_READLINE=1. This will start the main and debugger REPL in canonical\nterminal settings, which will allow use with rlwrap.
For example, the following can be added to a .bashrc file:
alias node=\"env NODE_NO_READLINE=1 rlwrap node\"\nIt is possible to create and run multiple REPL instances against a single\nrunning instance of Node.js that share a single global object but have\nseparate I/O interfaces.
The following example, for instance, provides separate REPLs on stdin, a Unix\nsocket, and a TCP socket:
const net = require('net');\nconst repl = require('repl');\nlet connections = 0;\n\nrepl.start({\n prompt: 'Node.js via stdin> ',\n input: process.stdin,\n output: process.stdout\n});\n\nnet.createServer((socket) => {\n connections += 1;\n repl.start({\n prompt: 'Node.js via Unix socket> ',\n input: socket,\n output: socket\n }).on('exit', () => {\n socket.end();\n });\n}).listen('/tmp/node-repl-sock');\n\nnet.createServer((socket) => {\n connections += 1;\n repl.start({\n prompt: 'Node.js via TCP socket> ',\n input: socket,\n output: socket\n }).on('exit', () => {\n socket.end();\n });\n}).listen(5001);\nRunning this application from the command line will start a REPL on stdin.\nOther REPL clients may connect through the Unix socket or TCP socket. telnet,\nfor instance, is useful for connecting to TCP sockets, while socat can be used\nto connect to both Unix and TCP sockets.
By starting a REPL from a Unix socket-based server instead of stdin, it is\npossible to connect to a long-running Node.js process without restarting it.
\nFor an example of running a \"full-featured\" (terminal) REPL over\na net.Server and net.Socket instance, see:\nhttps://gist.github.com/TooTallNate/2209310.
For an example of running a REPL instance over curl(1), see:\nhttps://gist.github.com/TooTallNate/2053342.
The repl.REPLServer class inherits from the readline.Interface class.\nInstances of repl.REPLServer are created using the repl.start() method and\nshould not be created directly using the JavaScript new keyword.
The 'exit' event is emitted when the REPL is exited either by receiving the\n.exit command as input, the user pressing <ctrl>-C twice to signal SIGINT,\nor by pressing <ctrl>-D to signal 'end' on the input stream. The listener\ncallback is invoked without any arguments.
replServer.on('exit', () => {\n console.log('Received \"exit\" event from repl!');\n process.exit();\n});\nThe 'reset' event is emitted when the REPL's context is reset. This occurs\nwhenever the .clear command is received as input unless the REPL is using\nthe default evaluator and the repl.REPLServer instance was created with the\nuseGlobal option set to true. The listener callback will be called with a\nreference to the context object as the only argument.
This can be used primarily to re-initialize REPL context to some pre-defined\nstate:
\nconst repl = require('repl');\n\nfunction initializeContext(context) {\n context.m = 'test';\n}\n\nconst r = repl.start({ prompt: '> ' });\ninitializeContext(r.context);\n\nr.on('reset', initializeContext);\nWhen this code is executed, the global 'm' variable can be modified but then\nreset to its initial value using the .clear command:
$ ./node example.js\n> m\n'test'\n> m = 1\n1\n> m\n1\n> .clear\nClearing context...\n> m\n'test'\n>\nThe replServer.defineCommand() method is used to add new .-prefixed commands\nto the REPL instance. Such commands are invoked by typing a . followed by the\nkeyword. The cmd is either a Function or an Object with the following\nproperties:
help <string> Help text to be displayed when .help is entered (Optional).action <Function> The function to execute, optionally accepting a single\nstring argument.The following example shows two new commands added to the REPL instance:
\nconst repl = require('repl');\n\nconst replServer = repl.start({ prompt: '> ' });\nreplServer.defineCommand('sayhello', {\n help: 'Say hello',\n action(name) {\n this.clearBufferedCommand();\n console.log(`Hello, ${name}!`);\n this.displayPrompt();\n }\n});\nreplServer.defineCommand('saybye', function saybye() {\n console.log('Goodbye!');\n this.close();\n});\nThe new commands can then be used from within the REPL instance:
\n> .sayhello Node.js User\nHello, Node.js User!\n> .saybye\nGoodbye!\nThe replServer.displayPrompt() method readies the REPL instance for input\nfrom the user, printing the configured prompt to a new line in the output\nand resuming the input to accept new input.
When multi-line input is being entered, an ellipsis is printed rather than the\n'prompt'.
\nWhen preserveCursor is true, the cursor placement will not be reset to 0.
The replServer.displayPrompt method is primarily intended to be called from\nwithin the action function for commands registered using the\nreplServer.defineCommand() method.
The replServer.clearBufferedCommand() method clears any command that has been\nbuffered but not yet executed. This method is primarily intended to be\ncalled from within the action function for commands registered using the\nreplServer.defineCommand() method.
An internal method used to parse and execute REPLServer keywords.\nReturns true if keyword is a valid keyword, otherwise false.
The repl.start() method creates and starts a repl.REPLServer instance.
If options is a string, then it specifies the input prompt:
const repl = require('repl');\n\n// a Unix style prompt\nrepl.start('$ ');\n