repl

Deno compatibility

All symbols are not supported.

The node:repl 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:

import repl from 'node:repl';

Usage in Deno

import * as mod from "node:repl";

class Recoverable

extends SyntaxError

Usage in Deno

import { Recoverable } from "node:repl";

Deno compatibility

This symbol is not supported.

Indicates a recoverable error that a REPLServer can use to support multi-line input.

Constructors

Recoverable(err: Error)

new

Properties

err: Error

See

https://nodejs.org/dist/latest-v22.x/docs/api/repl.html#repl_recoverable_errors

class REPLServer

extends Interface

Usage in Deno

import { REPLServer } from "node:repl";

Deno compatibility

This symbol is not supported.

Instances of repl.REPLServer are created using the start method or directly using the JavaScript new keyword.

import repl from 'node:repl';

const options = { useColors: true };

const firstInstance = repl.start(options);
const secondInstance = new repl.REPLServer(options);

Constructors

REPLServer()

new

NOTE: According to the documentation:

Instances of repl.REPLServer are created using the repl.start() method and should not be created directly using the JavaScript new keyword.

REPLServer cannot be subclassed due to implementation specifics in NodeJS.

Properties

commands: ReadOnlyDict<REPLCommand>

readonly

The commands registered via replServer.defineCommand().

completer: Completer | AsyncCompleter

readonly

Specified in the REPL options, this is the function to use for custom Tab auto-completion.

context: Context

readonly

The vm.Context provided to the eval function to be used for JavaScript evaluation.

editorMode: boolean

readonly

A value indicating whether the REPL is currently in "editor mode".

eval: REPLEval

readonly

Specified in the REPL options, this is the function to be used when evaluating each given line of input. If not specified in the REPL options, this is an async wrapper for the JavaScript eval() function.

ignoreUndefined: boolean

readonly

Specified in the REPL options, this is a value indicating whether the default writer function should output the result of a command if it evaluates to undefined.

input: ReadableStream

readonly

The Readable stream from which REPL input will be read.

inputStream: ReadableStream

deprecated

readonly

last: any

readonly

The last evaluation result from the REPL (assigned to the _ variable inside of the REPL).

lastError: any

readonly

The last error raised inside the REPL (assigned to the _error variable inside of the REPL).

output: WritableStream

readonly

The Writable stream to which REPL output will be written.

outputStream: WritableStream

deprecated

readonly

replMode: REPL_MODE_SLOPPY | REPL_MODE_STRICT

readonly

Specified in the REPL options, this is a flag that specifies whether the default eval function should execute all JavaScript commands in strict mode or default (sloppy) mode. Possible values are:

repl.REPL_MODE_SLOPPY - evaluates expressions in sloppy mode.
repl.REPL_MODE_STRICT - evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.

underscoreAssigned: boolean

readonly

A value indicating whether the _ variable has been assigned.

underscoreErrAssigned: boolean

readonly

A value indicating whether the _error variable has been assigned.

useColors: boolean

readonly

Specified in the REPL options, this is a value indicating whether the default writer function should include ANSI color styling to REPL output.

useGlobal: boolean

readonly

Specified in the REPL options, this is a value indicating whether the default eval function will use the JavaScript global as the context as opposed to creating a new separate context for the REPL instance.

writer: REPLWriter

readonly

Specified in the REPL options, this is the function to invoke to format the output of each command before writing to outputStream. If not specified in the REPL options, this will be a wrapper for util.inspect.

Methods

addListener(

event: string,

listener: (...args: any[]) => void,
): this

events.EventEmitter

close - inherited from readline.Interface
line - inherited from readline.Interface
pause - inherited from readline.Interface
resume - inherited from readline.Interface
SIGCONT - inherited from readline.Interface
SIGINT - inherited from readline.Interface
SIGTSTP - inherited from readline.Interface
exit
reset

addListener(

event: "close",

listener: () => void,
): this

addListener(

event: "line",

listener: (input: string) => void,
): this

addListener(

event: "pause",

listener: () => void,
): this

addListener(

event: "resume",

listener: () => void,
): this

addListener(

event: "SIGCONT",

listener: () => void,
): this

addListener(

event: "SIGINT",

listener: () => void,
): this

addListener(

event: "SIGTSTP",

listener: () => void,
): this

addListener(

event: "exit",

listener: () => void,
): this

addListener(

event: "reset",

listener: (context: Context) => void,
): this

clearBufferedCommand(): void

The replServer.clearBufferedCommand() 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 replServer.defineCommand() method.

defineCommand(

keyword: string,

cmd: REPLCommandAction | REPLCommand,
): void

The replServer.defineCommand() method is used to add new .-prefixed commands to the REPL instance. Such commands are invoked by typing a . followed by the keyword. The cmd is either a Function or an Object with the following properties:

The following example shows two new commands added to the REPL instance:

import repl from 'node: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();
});

The new commands can then be used from within the REPL instance:

> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!

displayPrompt(preserveCursor?: boolean): void

The replServer.displayPrompt() method readies the REPL instance for input from the user, printing the configured prompt to a new line in the output and resuming the input to accept new input.

When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.

When preserveCursor is true, the cursor placement will not be reset to 0.

The replServer.displayPrompt method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

emit(

event: string | symbol,

...args: any[],
): boolean

emit(event: "close"): boolean

emit(

event: "line",

input: string,
): boolean

emit(event: "pause"): boolean

emit(event: "resume"): boolean

emit(event: "SIGCONT"): boolean

emit(event: "SIGINT"): boolean

emit(event: "SIGTSTP"): boolean

emit(event: "exit"): boolean

emit(

event: "reset",

context: Context,
): boolean

on(

event: string,

listener: (...args: any[]) => void,
): this

on(

event: "close",

listener: () => void,
): this

on(

event: "line",

listener: (input: string) => void,
): this

on(

event: "pause",

listener: () => void,
): this

on(

event: "resume",

listener: () => void,
): this

on(

event: "SIGCONT",

listener: () => void,
): this

on(

event: "SIGINT",

listener: () => void,
): this

on(

event: "SIGTSTP",

listener: () => void,
): this

on(

event: "exit",

listener: () => void,
): this

on(

event: "reset",

listener: (context: Context) => void,
): this

once(

event: string,

listener: (...args: any[]) => void,
): this

once(

event: "close",

listener: () => void,
): this

once(

event: "line",

listener: (input: string) => void,
): this

once(

event: "pause",

listener: () => void,
): this

once(

event: "resume",

listener: () => void,
): this

once(

event: "SIGCONT",

listener: () => void,
): this

once(

event: "SIGINT",

listener: () => void,
): this

once(

event: "SIGTSTP",

listener: () => void,
): this

once(

event: "exit",

listener: () => void,
): this

once(

event: "reset",

listener: (context: Context) => void,
): this

prependListener(

event: string,

listener: (...args: any[]) => void,
): this

prependListener(

event: "close",

listener: () => void,
): this

prependListener(

event: "line",

listener: (input: string) => void,
): this

prependListener(

event: "pause",

listener: () => void,
): this

prependListener(

event: "resume",

listener: () => void,
): this

prependListener(

event: "SIGCONT",

listener: () => void,
): this

prependListener(

event: "SIGINT",

listener: () => void,
): this

prependListener(

event: "SIGTSTP",

listener: () => void,
): this

prependListener(

event: "exit",

listener: () => void,
): this

prependListener(

event: "reset",

listener: (context: Context) => void,
): this

prependOnceListener(

event: string,

listener: (...args: any[]) => void,
): this

prependOnceListener(

event: "close",

listener: () => void,
): this

prependOnceListener(

event: "line",

listener: (input: string) => void,
): this

prependOnceListener(

event: "pause",

listener: () => void,
): this

prependOnceListener(

event: "resume",

listener: () => void,
): this

prependOnceListener(

event: "SIGCONT",

listener: () => void,
): this

prependOnceListener(

event: "SIGINT",

listener: () => void,
): this

prependOnceListener(

event: "SIGTSTP",

listener: () => void,
): this

prependOnceListener(

event: "exit",

listener: () => void,
): this

prependOnceListener(

event: "reset",

listener: (context: Context) => void,
): this

setupHistory(

path: string,

callback: (

err: Error | null,

repl: this,
) => void,
): void

Initializes a history log file for the REPL instance. When executing the Node.js binary and using the command-line REPL, a history file is initialized by default. However, this is not the case when creating a REPL programmatically. Use this method to initialize a history log file when working with REPL instances programmatically.

function start

Usage in Deno

import { start } from "node:repl";

start(options?: string | ReplOptions): REPLServer

Deno compatibility

This symbol is not supported.

The repl.start() method creates and starts a REPLServer instance.

If options is a string, then it specifies the input prompt:

import repl from 'node:repl';

// a Unix style prompt
repl.start('$ ');

Parameters

options: string | ReplOptions

optional

Return Type

REPLServer

interface REPLCommand

Usage in Deno

import { type REPLCommand } from "node:repl";

Deno compatibility

This symbol is not supported.

Properties

help: string | undefined

optional

Help text to be displayed when .help is entered.

action: REPLCommandAction

The function to execute, optionally accepting a single string argument.

interface ReplOptions

Usage in Deno

import { type ReplOptions } from "node:repl";

Deno compatibility

This symbol is not supported.

Properties

prompt: string | undefined

optional

The input prompt to display.

input: ReadableStream | undefined

optional

The Readable stream from which REPL input will be read.

output: WritableStream | undefined

optional

The Writable stream to which REPL output will be written.

terminal: boolean | undefined

optional

If true, specifies that the output should be treated as a TTY terminal, and have ANSI/VT100 escape codes written to it. Default: checking the value of the isTTY property on the output stream upon instantiation.

eval: REPLEval | undefined

optional

The function to be used when evaluating each given line of input. Default: an async wrapper for the JavaScript eval() function. An eval function can error with repl.Recoverable to indicate the input was incomplete and prompt for additional lines.

preview: boolean | undefined

optional

Defines if the repl prints output previews or not.

useColors: boolean | undefined

optional

If true, specifies that the default writer function should include ANSI color styling to REPL output. If a custom writer function is provided then this has no effect.

useGlobal: boolean | undefined

optional

If true, specifies that the default evaluation function will use the JavaScript global as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value to true.

ignoreUndefined: boolean | undefined

optional

If true, specifies that the default writer will not output the return value of a command if it evaluates to undefined.

writer: REPLWriter | undefined

optional

The function to invoke to format the output of each command before writing to output.

completer: 
Completer
 | AsyncCompleter
 | undefined

optional

An optional function used for custom Tab auto completion.

replMode: 
REPL_MODE_SLOPPY
 | REPL_MODE_STRICT
 | undefined

optional

A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Accepted values are:

repl.REPL_MODE_SLOPPY - evaluates expressions in sloppy mode.
repl.REPL_MODE_STRICT - evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.

breakEvalOnSigint: boolean | undefined

optional

Stop evaluating the current piece of code when SIGINT is received, i.e. Ctrl+C is pressed. This cannot be used together with a custom eval function.

type alias REPLCommandAction

Usage in Deno

import { type REPLCommandAction } from "node:repl";

Deno compatibility

This symbol is not supported.

Definition

(

this: REPLServer,

text: string,
) => void

type alias REPLEval

Usage in Deno

import { type REPLEval } from "node:repl";

Deno compatibility

This symbol is not supported.

Definition

(

this: REPLServer,

evalCmd: string,

context: Context,

file: string,

cb: (

err: Error | null,

result: any,
) => void,
) => void

type alias REPLWriter

Usage in Deno

import { type REPLWriter } from "node:repl";

Deno compatibility

This symbol is not supported.

Definition

(

this: REPLServer,

obj: any,
) => string

variable REPL_MODE_SLOPPY

Usage in Deno

import { REPL_MODE_SLOPPY } from "node:repl";

A flag passed in the REPL options. Evaluates expressions in sloppy mode.

Type

unique symbol

variable REPL_MODE_STRICT

Usage in Deno

import { REPL_MODE_STRICT } from "node:repl";

A flag passed in the REPL options. Evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.

Type

unique symbol

variable writer

Usage in Deno

import { writer } from "node:repl";

This is the default "writer" value, if none is passed in the REPL options, and it can be overridden by custom print functions.

Type

REPLWriter & { options: InspectOptions; }