Accept input from the command line in Node.js
How to make a Node.js CLI program interactive?
Node.js since version 7 provides the readline
module to perform exactly this: get input from a readable stream such as the process.stdin
stream, which during the execution of a Node.js program is the terminal input, one line at a time.
const module "node:readline"
readline = var require: NodeJS.Require
(id: string) => any
Used to import modules, `JSON`, and local files.require('node:readline');
const const rl: readline.Interface
rl = module "node:readline"
readline.function createInterface(options: readline.ReadLineOptions): readline.Interface (+1 overload)
The `readline.createInterface()` method creates a new `readline.Interface` instance.
```js
import readline from 'node:readline';
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
});
```
Once the `readline.Interface` instance is created, the most common case is to
listen for the `'line'` event:
```js
rl.on('line', (line) => {
console.log(`Received: ${line}`);
});
```
If `terminal` is `true` for this instance then the `output` stream will get
the best compatibility if it defines an `output.columns` property and emits
a `'resize'` event on the `output` if or when the columns ever change
(`process.stdout` does this automatically when it is a TTY).
When creating a `readline.Interface` using `stdin` as input, the program
will not terminate until it receives an [EOF character](https://en.wikipedia.org/wiki/End-of-file#EOF_character). To exit without
waiting for user input, call `process.stdin.unref()`.createInterface({
ReadLineOptions.input: NodeJS.ReadableStream
The [`Readable`](https://nodejs.org/docs/latest-v22.x/api/stream.html#readable-streams) stream to listen toinput: var process: NodeJS.Process
process.NodeJS.Process.stdin: NodeJS.ReadStream & {
fd: 0;
}
The `process.stdin` property returns a stream connected to`stdin` (fd `0`). It is a `net.Socket` (which is a `Duplex` stream) unless fd `0` refers to a file, in which case it is
a `Readable` stream.
For details of how to read from `stdin` see `readable.read()`.
As a `Duplex` stream, `process.stdin` can also be used in "old" mode that
is compatible with scripts written for Node.js prior to v0.10\.
For more information see `Stream compatibility`.
In "old" streams mode the `stdin` stream is paused by default, so one
must call `process.stdin.resume()` to read from it. Note also that calling `process.stdin.resume()` itself would switch stream to "old" mode.stdin,
ReadLineOptions.output?: NodeJS.WritableStream | undefined
The [`Writable`](https://nodejs.org/docs/latest-v22.x/api/stream.html#writable-streams) stream to write readline data to.output: var process: NodeJS.Process
process.NodeJS.Process.stdout: NodeJS.WriteStream & {
fd: 1;
}
The `process.stdout` property returns a stream connected to`stdout` (fd `1`). It is a `net.Socket` (which is a `Duplex` stream) unless fd `1` refers to a file, in which case it is
a `Writable` stream.
For example, to copy `process.stdin` to `process.stdout`:
```js
import { stdin, stdout } from 'node:process';
stdin.pipe(stdout);
```
`process.stdout` differs from other Node.js streams in important ways. See `note on process I/O` for more information.stdout,
});
const rl: readline.Interface
rl.Interface.question(query: string, callback: (answer: string) => void): void (+1 overload)
The `rl.question()` method displays the `query` by writing it to the `output`,
waits for user input to be provided on `input`, then invokes the `callback` function passing the provided input as the first argument.
When called, `rl.question()` will resume the `input` stream if it has been
paused.
If the `Interface` was created with `output` set to `null` or `undefined` the `query` is not written.
The `callback` function passed to `rl.question()` does not follow the typical
pattern of accepting an `Error` object or `null` as the first argument.
The `callback` is called with the provided answer as the only argument.
An error will be thrown if calling `rl.question()` after `rl.close()`.
Example usage:
```js
rl.question('What is your favorite food? ', (answer) => {
console.log(`Oh, so your favorite food is ${answer}`);
});
```
Using an `AbortController` to cancel a question.
```js
const ac = new AbortController();
const signal = ac.signal;
rl.question('What is your favorite food? ', { signal }, (answer) => {
console.log(`Oh, so your favorite food is ${answer}`);
});
signal.addEventListener('abort', () => {
console.log('The food question timed out');
}, { once: true });
setTimeout(() => ac.abort(), 10000);
```question(`What's your name?`, name: string
name => {
var console: Console
The `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)
Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log(`Hi ${name: string
name}!`);
const rl: readline.Interface
rl.Interface.close(): void
The `rl.close()` method closes the `Interface` instance and
relinquishes control over the `input` and `output` streams. When called,
the `'close'` event will be emitted.
Calling `rl.close()` does not immediately stop other events (including `'line'`)
from being emitted by the `Interface` instance.close();
});
This piece of code asks the user's name, and once the text is entered and the user presses enter, we send a greeting.
The question()
method shows the first parameter (a question) and waits for the user input. It calls the callback function once enter is pressed.
In this callback function, we close the readline interface.
readline
offers several other methods, please check them out on the package documentation linked above.
If you need to require a password, it's best not to echo it back, but instead show a *
symbol.