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Node.js v5.7.1 Documentation
Table of Contents
Readline#
Stability: 2 - Stable
To use this module, do require('readline')
. Readline allows reading of a
stream (such as process.stdin
) on a line-by-line basis.
Note that once you've invoked this module, your Node.js program will not terminate until you've closed the interface. Here's how to allow your program to gracefully exit:
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();
});
Class: Interface#
The class that represents a readline interface with an input and output stream.
rl.close()#
Closes the Interface
instance, relinquishing control on the input
and
output
streams. The 'close'
event will also be emitted.
rl.pause()#
Pauses the readline input
stream, allowing it to be resumed later if needed.
Note that this doesn't immediately pause the stream of events. Several events may
be emitted after calling pause
, including line
.
rl.prompt([preserveCursor])#
Readies readline for input from the user, putting the current setPrompt
options on a new line, giving the user a new spot to write. Set preserveCursor
to true
to prevent the cursor placement being reset to 0
.
This will also resume the input
stream used with createInterface
if it has
been paused.
If output
is set to null
or undefined
when calling createInterface
, the
prompt is not written.
rl.question(query, callback)#
Prepends the prompt with query
and invokes callback
with the user's
response. Displays the query to the user, and then invokes callback
with the user's response after it has been typed.
This will also resume the input
stream used with createInterface
if
it has been paused.
If output
is set to null
or undefined
when calling createInterface
,
nothing is displayed.
Example usage:
rl.question('What is your favorite food?', (answer) => {
console.log(`Oh, so your favorite food is ${answer}`);
});
rl.resume()#
Resumes the readline input
stream.
rl.setPrompt(prompt)#
Sets the prompt, for example when you run node
on the command line, you see
>
, which is Node.js's prompt.
rl.write(data[, key])#
Writes data
to output
stream, unless output
is set to null
or
undefined
when calling createInterface
. key
is an object literal to
represent a key sequence; available if the terminal is a TTY.
This will also resume the input
stream if it has been paused.
Example:
rl.write('Delete me!');
// Simulate ctrl+u to delete the line written previously
rl.write(null, {ctrl: true, name: 'u'});
Events#
Event: 'close'#
function () {}
Emitted when close()
is called.
Also emitted when the input
stream receives its 'end'
event. The Interface
instance should be considered "finished" once this is emitted. For example, when
the input
stream receives ^D
, respectively known as EOT
.
This event is also called if there is no SIGINT
event listener present when
the input
stream receives a ^C
, respectively known as SIGINT
.
Event: 'line'#
function (line) {}
Emitted whenever the input
stream receives an end of line (\n
, \r
, or
\r\n
), usually received when the user hits enter, or return. This is a good
hook to listen for user input.
Example of listening for 'line'
:
rl.on('line', (cmd) => {
console.log(`You just typed: ${cmd}`);
});
Event: 'pause'#
function () {}
Emitted whenever the input
stream is paused.
Also emitted whenever the input
stream is not paused and receives the
SIGCONT
event. (See events SIGTSTP
and SIGCONT
)
Example of listening for 'pause'
:
rl.on('pause', () => {
console.log('Readline paused.');
});
Event: 'resume'#
function () {}
Emitted whenever the input
stream is resumed.
Example of listening for 'resume'
:
rl.on('resume', () => {
console.log('Readline resumed.');
});
Event: 'SIGCONT'#
function () {}
This does not work on Windows.
Emitted whenever the input
stream is sent to the background with ^Z
,
respectively known as SIGTSTP
, and then continued with fg(1)
. This event
only emits if the stream was not paused before sending the program to the
background.
Example of listening for SIGCONT
:
rl.on('SIGCONT', () => {
// `prompt` will automatically resume the stream
rl.prompt();
});
Event: 'SIGINT'#
function () {}
Emitted whenever the input
stream receives a ^C
, respectively known as
SIGINT
. If there is no SIGINT
event listener present when the input
stream receives a SIGINT
, pause
will be triggered.
Example of listening for SIGINT
:
rl.on('SIGINT', () => {
rl.question('Are you sure you want to exit?', (answer) => {
if (answer.match(/^y(es)?$/i)) rl.pause();
});
});
Event: 'SIGTSTP'#
function () {}
This does not work on Windows.
Emitted whenever the input
stream receives a ^Z
, respectively known as
SIGTSTP
. If there is no SIGTSTP
event listener present when the input
stream receives a SIGTSTP
, the program will be sent to the background.
When the program is resumed with fg
, the 'pause'
and SIGCONT
events will be
emitted. You can use either to resume the stream.
The 'pause'
and SIGCONT
events will not be triggered if the stream was paused
before the program was sent to the background.
Example of listening for SIGTSTP
:
rl.on('SIGTSTP', () => {
// This will override SIGTSTP and prevent the program from going to the
// background.
console.log('Caught SIGTSTP.');
});
Example: Tiny CLI#
Here's an example of how to use all these together to craft a tiny command line interface:
const readline = require('readline');
const rl = readline.createInterface(process.stdin, process.stdout);
rl.setPrompt('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 case for readline
's input
option is to pass a filesystem readable
stream to it. This is how one could craft line-by-line parsing of a file:
const readline = require('readline');
const fs = require('fs');
const rl = readline.createInterface({
input: fs.createReadStream('sample.txt')
});
rl.on('line', (line) => {
console.log('Line from file:', line);
});
readline.clearLine(stream, dir)#
Clears current line of given TTY stream in a specified direction.
dir
should have one of following values:
-1
- to the left from cursor1
- to the right from cursor0
- the entire line
readline.clearScreenDown(stream)#
Clears the screen from the current position of the cursor down.
readline.createInterface(options)#
Creates a readline Interface
instance. Accepts an options
Object that takes
the following values:
input
- the readable stream to listen to (Required).output
- the writable stream to write readline data to (Optional).completer
- an optional function that is used for Tab autocompletion. See below for an example of using this.terminal
- passtrue
if theinput
andoutput
streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Defaults to checkingisTTY
on theoutput
stream upon instantiation.historySize
- maximum number of history lines retained. Defaults to30
.
The completer
function is given the current line entered by the user, and
is supposed to return an Array with 2 entries:
An Array with matching entries for the completion.
The substring that was used for the matching.
Which ends up looking something like:
[[substr1, substr2, ...], originalsubstring]
.
Example:
function completer(line) {
var completions = '.help .error .exit .quit .q'.split(' ')
var hits = completions.filter((c) => { return c.indexOf(line) == 0 })
// show all completions if none found
return [hits.length ? hits : completions, line]
}
Also completer
can be run in async mode if it accepts two arguments:
function completer(linePartial, callback) {
callback(null, [['123'], linePartial]);
}
createInterface
is commonly used with process.stdin
and
process.stdout
in order to accept user input:
const readline = require('readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
Once you have a readline instance, you most commonly listen for the
'line'
event.
If terminal
is true
for this instance then the output
stream will get
the best compatibility if it defines an output.columns
property, and fires
a 'resize'
event on the output
if/when the columns ever change
(process.stdout
does this automatically when it is a TTY).
readline.cursorTo(stream, x, y)#
Move cursor to the specified position in a given TTY stream.
readline.moveCursor(stream, dx, dy)#
Move cursor relative to it's current position in a given TTY stream.