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|
# 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
<!-- YAML
added: v0.1.104
-->
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'
<!-- YAML
added: v0.1.98
-->
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'
<!-- YAML
added: v0.1.98
-->
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: ${input}`);
});
```
### Event: 'pause'
<!-- YAML
added: v0.7.5
-->
The `'pause'` event is emitted when one of the following occur:
* The `input` stream is paused.
* The `input` stream is not paused and receives the `'SIGCONT'` event. (See
events [`'SIGTSTP'`][] and [`'SIGCONT'`][].)
The listener function is called without passing any arguments.
```js
rl.on('pause', () => {
console.log('Readline paused.');
});
```
### Event: 'resume'
<!-- YAML
added: v0.7.5
-->
The `'resume'` event is emitted whenever the `input` stream is resumed.
The listener function is called without passing any arguments.
```js
rl.on('resume', () => {
console.log('Readline resumed.');
});
```
### Event: 'SIGCONT'
<!-- YAML
added: v0.7.5
-->
The `'SIGCONT'` event is emitted when a Node.js process previously moved into
the background using `<ctrl>-Z` (i.e. `SIGTSTP`) is then brought back to the
foreground using fg(1p).
If the `input` stream was paused *before* the `SIGTSTP` request, this event will
not be emitted.
The listener function is invoked without passing any arguments.
```js
rl.on('SIGCONT', () => {
// `prompt` will automatically resume the stream
rl.prompt();
});
```
The `'SIGCONT'` event is _not_ supported on Windows.
### Event: 'SIGINT'
<!-- YAML
added: v0.3.0
-->
The `'SIGINT'` event is emitted whenever the `input` stream receives a
`<ctrl>-C` input, known typically as `SIGINT`. If there are no `'SIGINT'` event
listeners registered when the `input` stream receives a `SIGINT`, the `'pause'`
event will be emitted.
The listener function is invoked without passing any arguments.
```js
rl.on('SIGINT', () => {
rl.question('Are you sure you want to exit? ', (answer) => {
if (answer.match(/^y(es)?$/i)) rl.pause();
});
});
```
### Event: 'SIGTSTP'
<!-- YAML
added: v0.7.5
-->
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()
<!-- YAML
added: v0.1.98
-->
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()
<!-- YAML
added: v0.3.4
-->
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])
<!-- YAML
added: v0.1.98
-->
* `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)
<!-- YAML
added: v0.3.3
-->
* `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()
<!-- YAML
added: v0.3.4
-->
The `rl.resume()` method resumes the `input` stream if it has been paused.
### rl.setPrompt(prompt)
<!-- YAML
added: v0.1.98
-->
* `prompt` {string}
The `rl.setPrompt()` method sets the prompt that will be written to `output`
whenever `rl.prompt()` is called.
### rl.write(data[, key])
<!-- YAML
added: v0.1.98
-->
* `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*.
## readline.clearLine(stream, dir)
<!-- YAML
added: v0.7.7
-->
* `stream` {stream.Writable}
* `dir` {number}
* `-1` - to the left from cursor
* `1` - to the right from cursor
* `0` - the entire line
The `readline.clearLine()` method clears current line of given [TTY][] stream
in a specified direction identified by `dir`.
## readline.clearScreenDown(stream)
<!-- YAML
added: v0.7.7
-->
* `stream` {stream.Writable}
The `readline.clearScreenDown()` method clears the given [TTY][] stream from
the current position of the cursor down.
## readline.createInterface(options)
<!-- YAML
added: v0.1.98
changes:
- version: v8.3.0, 6.11.4
pr-url: https://github.com/nodejs/node/pull/13497
description: Remove max limit of `crlfDelay` option.
- 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.
-->
* `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)
<!-- YAML
added: v0.7.7
-->
* `stream` {stream.Writable}
* `x` {number}
* `y` {number}
The `readline.cursorTo()` method moves cursor to the specified position in a
given [TTY][] `stream`.
## readline.emitKeypressEvents(stream[, interface])
<!-- YAML
added: v0.7.7
-->
* `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)
<!-- YAML
added: v0.7.7
-->
* `stream` {stream.Writable}
* `dx` {number}
* `dy` {number}
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 input from a filesystem
[Readable][] stream one line at a time:
```js
const readline = require('readline');
const fs = require('fs');
const rl = readline.createInterface({
input: fs.createReadStream('sample.txt'),
crlfDelay: Infinity
});
rl.on('line', (line) => {
console.log(`Line from file: ${line}`);
});
```
[`'SIGCONT'`]: readline.html#readline_event_sigcont
[`'SIGTSTP'`]: readline.html#readline_event_sigtstp
[`process.stdin`]: process.html#process_process_stdin
[`process.stdout`]: process.html#process_process_stdout
[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
|