nodejs/doc/api/readline.md

# Readline

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

The `readline` module provides an interface for reading data from a [Readable][]
stream (such as [`process.stdin`][]) one line at a time. It can be accessed
using:

```js
const readline = require('readline');
```

The following simple example illustrates the basic use of the `readline` module.

```js
const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question('What do you think of Node.js? ', (answer) => {
  // TODO: Log the answer in a database
  console.log(`Thank you for your valuable feedback: ${answer}`);

  rl.close();
});
```

Once this code is invoked, the Node.js application will not terminate until the
`readline.Interface` is closed because the interface waits for data to be
received on the `input` stream.

## Class: `Interface`
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* Extends: {EventEmitter}

Instances of the `readline.Interface` class are constructed using the
`readline.createInterface()` method. Every instance is associated with a
single `input` [Readable][] stream and a single `output` [Writable][] stream.
The `output` stream is used to print prompts for user input that arrives on,
and is read from, the `input` stream.

### Event: `'close'`
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The `'close'` event is emitted when one of the following occur:

* The `rl.close()` method is called and the `readline.Interface` instance has
  relinquished control over the `input` and `output` streams;
* The `input` stream receives its `'end'` event;
* The `input` stream receives `<ctrl>-D` to signal end-of-transmission (EOT);
* The `input` stream receives `<ctrl>-C` to signal `SIGINT` and there is no
  `'SIGINT'` event listener registered on the `readline.Interface` instance.

The listener function is called without passing any arguments.

The `readline.Interface` instance is finished once the `'close'` event is
emitted.

### Event: `'line'`
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The `'line'` event is emitted whenever the `input` stream receives an
end-of-line input (`\n`, `\r`, or `\r\n`). This usually occurs when the user
presses the `<Enter>`, or `<Return>` keys.

The listener function is called with a string containing the single line of
received input.

```js
rl.on('line', (input) => {
  console.log(`Received: ${input}`);
});
```

### Event: `'pause'`
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The `'pause'` event is emitted when one of the following occur:

* The `input` stream is paused.
* The `input` stream is not paused and receives the `'SIGCONT'` event. (See
  events [`'SIGTSTP'`][] and [`'SIGCONT'`][].)

The listener function is called without passing any arguments.

```js
rl.on('pause', () => {
  console.log('Readline paused.');
});
```

### Event: `'resume'`
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The `'resume'` event is emitted whenever the `input` stream is resumed.

The listener function is called without passing any arguments.

```js
rl.on('resume', () => {
  console.log('Readline resumed.');
});
```

### Event: `'SIGCONT'`
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The `'SIGCONT'` event is emitted when a Node.js process previously moved into
the background using `<ctrl>-Z` (i.e. `SIGTSTP`) is then brought back to the
foreground using fg(1p).

If the `input` stream was paused *before* the `SIGTSTP` request, this event will
not be emitted.

The listener function is invoked without passing any arguments.

```js
rl.on('SIGCONT', () => {
  // `prompt` will automatically resume the stream
  rl.prompt();
});
```

The `'SIGCONT'` event is _not_ supported on Windows.

### Event: `'SIGINT'`
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The `'SIGINT'` event is emitted whenever the `input` stream receives a
`<ctrl>-C` input, known typically as `SIGINT`. If there are no `'SIGINT'` event
listeners registered when the `input` stream receives a `SIGINT`, the `'pause'`
event will be emitted.

The listener function is invoked without passing any arguments.

```js
rl.on('SIGINT', () => {
  rl.question('Are you sure you want to exit? ', (answer) => {
    if (answer.match(/^y(es)?$/i)) rl.pause();
  });
});
```

### Event: `'SIGTSTP'`
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The `'SIGTSTP'` event is emitted when the `input` stream receives a `<ctrl>-Z`
input, typically known as `SIGTSTP`. If there are no `'SIGTSTP'` event listeners
registered when the `input` stream receives a `SIGTSTP`, the Node.js process
will be sent to the background.

When the program is resumed using fg(1p), the `'pause'` and `'SIGCONT'` events
will be emitted. These can be used to resume the `input` stream.

The `'pause'` and `'SIGCONT'` events will not be emitted if the `input` was
paused before the process was sent to the background.

The listener function is invoked without passing any arguments.

```js
rl.on('SIGTSTP', () => {
  // This will override SIGTSTP and prevent the program from going to the
  // background.
  console.log('Caught SIGTSTP.');
});
```

The `'SIGTSTP'` event is _not_ supported on Windows.

### `rl.close()`
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The `rl.close()` method closes the `readline.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 `readline.Interface` instance.

### `rl.pause()`
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The `rl.pause()` method pauses the `input` stream, allowing it to be resumed
later if necessary.

Calling `rl.pause()` does not immediately pause other events (including
`'line'`) from being emitted by the `readline.Interface` instance.

### `rl.prompt([preserveCursor])`
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* `preserveCursor` {boolean} If `true`, prevents the cursor placement from
  being reset to `0`.

The `rl.prompt()` method writes the `readline.Interface` instances configured
`prompt` to a new line in `output` in order to provide a user with a new
location at which to provide input.

When called, `rl.prompt()` will resume the `input` stream if it has been
paused.

If the `readline.Interface` was created with `output` set to `null` or
`undefined` the prompt is not written.

### `rl.question(query, callback)`
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* `query` {string} A statement or query to write to `output`, prepended to the
  prompt.
* `callback` {Function} A callback function that is invoked with the user's
  input in response to the `query`.

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 `readline.Interface` was created with `output` set to `null` or
`undefined` the `query` is not written.

Example usage:

```js
rl.question('What is your favorite food? ', (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
});
```

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.

### `rl.resume()`
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The `rl.resume()` method resumes the `input` stream if it has been paused.

### `rl.setPrompt(prompt)`
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* `prompt` {string}

The `rl.setPrompt()` method sets the prompt that will be written to `output`
whenever `rl.prompt()` is called.

### `rl.write(data[, key])`
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* `data` {string}
* `key` {Object}
  * `ctrl` {boolean} `true` to indicate the `<ctrl>` key.
  * `meta` {boolean} `true` to indicate the `<Meta>` key.
  * `shift` {boolean} `true` to indicate the `<Shift>` key.
  * `name` {string} The name of the a key.

The `rl.write()` method will write either `data` or a key sequence identified
by `key` to the `output`. The `key` argument is supported only if `output` is
a [TTY][] text terminal.

If `key` is specified, `data` is ignored.

When called, `rl.write()` will resume the `input` stream if it has been
paused.

If the `readline.Interface` was created with `output` set to `null` or
`undefined` the `data` and `key` are not written.

```js
rl.write('Delete this!');
// Simulate Ctrl+u to delete the line written previously
rl.write(null, { ctrl: true, name: 'u' });
```

The `rl.write()` method will write the data to the `readline` `Interface`'s
`input` *as if it were provided by the user*.

### `rl[Symbol.asyncIterator]()`
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* Returns: {AsyncIterator}

Create an `AsyncIterator` object that iterates through each line in the input
stream as a string. This method allows asynchronous iteration of
`readline.Interface` objects through `for await...of` loops.

Errors in the input stream are not forwarded.

If the loop is terminated with `break`, `throw`, or `return`,
[`rl.close()`][] will be called. In other words, iterating over a
`readline.Interface` will always consume the input stream fully.

Performance is not on par with the traditional `'line'` event API. Use `'line'`
instead for performance-sensitive applications.

```js
async function processLineByLine() {
  const rl = readline.createInterface({
    // ...
  });

  for await (const line of rl) {
    // Each line in the readline input will be successively available here as
    // `line`.
  }
}
```

### `rl.line`
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* {string|undefined}

The current input data being processed by node.

This can be used when collecting input from a TTY stream to retrieve the
current value that has been processed thus far, prior to the `line` event
being emitted.  Once the `line` event has been emitted, this property will
be an empty string.

Be aware that modifying the value during the instance runtime may have
unintended consequences if `rl.cursor` is not also controlled.

**If not using a TTY stream for input, use the [`'line'`][] event.**

One possible use case would be as follows:

```js
const values = ['lorem ipsum', 'dolor sit amet'];
const rl = readline.createInterface(process.stdin);
const showResults = debounce(() => {
  console.log(
    '\n',
    values.filter((val) => val.startsWith(rl.line)).join(' ')
  );
}, 300);
process.stdin.on('keypress', (c, k) => {
  showResults();
});
```

### `rl.cursor`
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* {number|undefined}

The cursor position relative to `rl.line`.

This will track where the current cursor lands in the input string, when
reading input from a TTY stream.  The position of cursor determines the
portion of the input string that will be modified as input is processed,
as well as the column where the terminal caret will be rendered.

### `rl.getCursorPos()`
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* Returns: {Object}
  * `rows` {number} the row of the prompt the cursor currently lands on
  * `cols` {number} the screen column the cursor currently lands on

Returns the real position of the cursor in relation to the input
prompt + string.  Long input (wrapping) strings, as well as multiple
line prompts are included in the calculations.

## `readline.clearLine(stream, dir[, callback])`
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* `stream` {stream.Writable}
* `dir` {number}
  * `-1`: to the left from cursor
  * `1`: to the right from cursor
  * `0`: the entire line
* `callback` {Function} Invoked once the operation completes.
* Returns: {boolean} `false` if `stream` wishes for the calling code to wait for
  the `'drain'` event to be emitted before continuing to write additional data;
  otherwise `true`.

The `readline.clearLine()` method clears current line of given [TTY][] stream
in a specified direction identified by `dir`.

## `readline.clearScreenDown(stream[, callback])`
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* `stream` {stream.Writable}
* `callback` {Function} Invoked once the operation completes.
* Returns: {boolean} `false` if `stream` wishes for the calling code to wait for
  the `'drain'` event to be emitted before continuing to write additional data;
  otherwise `true`.

The `readline.clearScreenDown()` method clears the given [TTY][] stream from
the current position of the cursor down.

## `readline.createInterface(options)`
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  - version: v6.6.0
    pr-url: https://github.com/nodejs/node/pull/8109
    description: The `crlfDelay` option is supported now.
  - version: v6.3.0
    pr-url: https://github.com/nodejs/node/pull/7125
    description: The `prompt` option is supported now.
  - version: v6.0.0
    pr-url: https://github.com/nodejs/node/pull/6352
    description: The `historySize` option can be `0` now.
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* `options` {Object}
  * `input` {stream.Readable} The [Readable][] stream to listen to. This option
    is *required*.
  * `output` {stream.Writable} The [Writable][] stream to write readline data
    to.
  * `completer` {Function} An optional function used for Tab autocompletion.
  * `terminal` {boolean} `true` if the `input` and `output` streams should be
    treated like a TTY, and have ANSI/VT100 escape codes written to it.
    **Default:** checking `isTTY` on the `output` stream upon instantiation.
  * `historySize` {number} Maximum number of history lines retained. To disable
    the history set this value to `0`. This option makes sense only if
    `terminal` is set to `true` by the user or by an internal `output` check,
    otherwise the history caching mechanism is not initialized at all.
    **Default:** `30`.
  * `prompt` {string} The prompt string to use. **Default:** `'> '`.
  * `crlfDelay` {number} If the delay between `\r` and `\n` exceeds
    `crlfDelay` milliseconds, both `\r` and `\n` will be treated as separate
    end-of-line input. `crlfDelay` will be coerced to a number no less than
    `100`. It can be set to `Infinity`, in which case `\r` followed by `\n`
    will always be considered a single newline (which may be reasonable for
    [reading files][] with `\r\n` line delimiter). **Default:** `100`.
  * `removeHistoryDuplicates` {boolean} If `true`, when a new input line added
    to the history list duplicates an older one, this removes the older line
    from the list. **Default:** `false`.
  * `escapeCodeTimeout` {number} The duration `readline` will wait for a
    character (when reading an ambiguous key sequence in milliseconds one that
    can both form a complete key sequence using the input read so far and can
    take additional input to complete a longer key sequence).
    **Default:** `500`.

The `readline.createInterface()` method creates a new `readline.Interface`
instance.

```js
const readline = require('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).

### Use of the `completer` Function

The `completer` function takes the current line entered by the user
as an argument, and returns an `Array` with 2 entries:

* An `Array` with matching entries for the completion.
* The substring that was used for the matching.

For instance: `[[substr1, substr2, ...], originalsubstring]`.

```js
function completer(line) {
  const completions = '.help .error .exit .quit .q'.split(' ');
  const hits = completions.filter((c) => c.startsWith(line));
  // Show all completions if none found
  return [hits.length ? hits : completions, line];
}
```

The `completer` function can be called asynchronously if it accepts two
arguments:

```js
function completer(linePartial, callback) {
  callback(null, [['123'], linePartial]);
}
```

## `readline.cursorTo(stream, x[, y][, callback])`
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* `stream` {stream.Writable}
* `x` {number}
* `y` {number}
* `callback` {Function} Invoked once the operation completes.
* Returns: {boolean} `false` if `stream` wishes for the calling code to wait for
  the `'drain'` event to be emitted before continuing to write additional data;
  otherwise `true`.

The `readline.cursorTo()` method moves cursor to the specified position in a
given [TTY][] `stream`.

## `readline.emitKeypressEvents(stream[, interface])`
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* `stream` {stream.Readable}
* `interface` {readline.Interface}

The `readline.emitKeypressEvents()` method causes the given [Readable][]
stream to begin emitting `'keypress'` events corresponding to received input.

Optionally, `interface` specifies a `readline.Interface` instance for which
autocompletion is disabled when copy-pasted input is detected.

If the `stream` is a [TTY][], then it must be in raw mode.

This is automatically called by any readline instance on its `input` if the
`input` is a terminal. Closing the `readline` instance does not stop
the `input` from emitting `'keypress'` events.

```js
readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
  process.stdin.setRawMode(true);
```

## `readline.moveCursor(stream, dx, dy[, callback])`
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* `stream` {stream.Writable}
* `dx` {number}
* `dy` {number}
* `callback` {Function} Invoked once the operation completes.
* Returns: {boolean} `false` if `stream` wishes for the calling code to wait for
  the `'drain'` event to be emitted before continuing to write additional data;
  otherwise `true`.

The `readline.moveCursor()` method moves the cursor *relative* to its current
position in a given [TTY][] `stream`.

## Example: Tiny CLI

The following example illustrates the use of `readline.Interface` class to
implement a small command-line interface:

```js
const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: 'OHAI> '
});

rl.prompt();

rl.on('line', (line) => {
  switch (line.trim()) {
    case 'hello':
      console.log('world!');
      break;
    default:
      console.log(`Say what? I might have heard '${line.trim()}'`);
      break;
  }
  rl.prompt();
}).on('close', () => {
  console.log('Have a great day!');
  process.exit(0);
});
```

## Example: Read File Stream Line-by-Line

A common use case for `readline` is to consume an input file one line at a
time. The easiest way to do so is leveraging the [`fs.ReadStream`][] API as
well as a `for await...of` loop:

```js
const fs = require('fs');
const readline = require('readline');

async function processLineByLine() {
  const fileStream = fs.createReadStream('input.txt');

  const rl = readline.createInterface({
    input: fileStream,
    crlfDelay: Infinity
  });
  // Note: we use the crlfDelay option to recognize all instances of CR LF
  // ('\r\n') in input.txt as a single line break.

  for await (const line of rl) {
    // Each line in input.txt will be successively available here as `line`.
    console.log(`Line from file: ${line}`);
  }
}

processLineByLine();
```

Alternatively, one could use the [`'line'`][] event:

```js
const fs = require('fs');
const readline = require('readline');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt'),
  crlfDelay: Infinity
});

rl.on('line', (line) => {
  console.log(`Line from file: ${line}`);
});
```

Currently, `for await...of` loop can be a bit slower. If `async` / `await`
flow and speed are both essential, a mixed approach can be applied:

```js
const { once } = require('events');
const { createReadStream } = require('fs');
const { createInterface } = require('readline');

(async function processLineByLine() {
  try {
    const rl = createInterface({
      input: createReadStream('big-file.txt'),
      crlfDelay: Infinity
    });

    rl.on('line', (line) => {
      // Process the line.
    });

    await once(rl, 'close');

    console.log('File processed.');
  } catch (err) {
    console.error(err);
  }
})();
```

[`'SIGCONT'`]: readline.html#readline_event_sigcont
[`'SIGTSTP'`]: readline.html#readline_event_sigtstp
[`'line'`]: #readline_event_line
[`fs.ReadStream`]: fs.html#fs_class_fs_readstream
[`process.stdin`]: process.html#process_process_stdin
[`process.stdout`]: process.html#process_process_stdout
[`rl.close()`]: #readline_rl_close
[Readable]: stream.html#stream_readable_streams
[TTY]: tty.html
[Writable]: stream.html#stream_writable_streams
[reading files]: #readline_example_read_file_stream_line_by_line