- Assertion Testing
- Async Hooks
- Buffer
- C++ Addons
- C/C++ Addons with N-API
- Child Processes
- Cluster
- Command Line Options
- Console
- Crypto
- Debugger
- Deprecated APIs
- DNS
- Domain
- ECMAScript Modules
- Errors
- Events
- File System
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules
- Net
- OS
- Path
- Performance Hooks
- Policies
- Process
- Punycode
- Query Strings
- Readline
- REPL
- Report
- Stream
- String Decoder
- Timers
- TLS/SSL
- Trace Events
- TTY
- UDP/Datagram
- URL
- Utilities
- V8
- VM
- Worker Threads
- Zlib
Node.js v13.2.1-nightly20191201cbd50262c0 Documentation
Table of Contents
Readline#
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:
const readline = require('readline');
The following simple example illustrates the basic use of the readline
module.
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#
- 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'#
The 'close'
event is emitted when one of the following occur:
- The
rl.close()
method is called and thereadline.Interface
instance has relinquished control over theinput
andoutput
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 signalSIGINT
and there is no'SIGINT'
event listener registered on thereadline.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'#
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.
rl.on('line', (input) => {
console.log(`Received: ${input}`);
});
Event: 'pause'#
The 'pause'
event is emitted when one of the following occur:
- The
input
stream is paused. - The
input
stream is not paused and receives the'SIGCONT'
event. (See events'SIGTSTP'
and'SIGCONT'
.)
The listener function is called without passing any arguments.
rl.on('pause', () => {
console.log('Readline paused.');
});
Event: 'resume'#
The 'resume'
event is emitted whenever the input
stream is resumed.
The listener function is called without passing any arguments.
rl.on('resume', () => {
console.log('Readline resumed.');
});
Event: 'SIGCONT'#
The 'SIGCONT'
event is emitted when a Node.js process previously moved into
the background using <ctrl>-Z
(i.e. SIGTSTP
) is then brought back to the
foreground using fg(1p)
.
If the input
stream was paused before the SIGTSTP
request, this event will
not be emitted.
The listener function is invoked without passing any arguments.
rl.on('SIGCONT', () => {
// `prompt` will automatically resume the stream
rl.prompt();
});
The 'SIGCONT'
event is not supported on Windows.
Event: 'SIGINT'#
The 'SIGINT'
event is emitted whenever the input
stream receives a
<ctrl>-C
input, known typically as SIGINT
. If there are no 'SIGINT'
event
listeners registered when the input
stream receives a SIGINT
, the 'pause'
event will be emitted.
The listener function is invoked without passing any arguments.
rl.on('SIGINT', () => {
rl.question('Are you sure you want to exit? ', (answer) => {
if (answer.match(/^y(es)?$/i)) rl.pause();
});
});
Event: 'SIGTSTP'#
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.
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()#
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()#
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])#
preserveCursor
<boolean> Iftrue
, prevents the cursor placement from being reset to0
.
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)#
query
<string> A statement or query to write tooutput
, prepended to the prompt.callback
<Function> A callback function that is invoked with the user's input in response to thequery
.
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:
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()#
The rl.resume()
method resumes the input
stream if it has been paused.
rl.setPrompt(prompt)#
prompt
<string>
The rl.setPrompt()
method sets the prompt that will be written to output
whenever rl.prompt()
is called.
rl.write(data[, 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.
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]()#
- 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.
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`.
}
}
readline.clearLine(stream, dir[, callback])[src]#
stream
<stream.Writable>-
dir
<number>-1
: to the left from cursor1
: to the right from cursor0
: the entire line
callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.clearLine()
method clears current line of given TTY stream
in a specified direction identified by dir
.
readline.clearScreenDown(stream[, callback])[src]#
stream
<stream.Writable>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.clearScreenDown()
method clears the given TTY stream from
the current position of the cursor down.
readline.createInterface(options)[src]#
-
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 theinput
andoutput
streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checkingisTTY
on theoutput
stream upon instantiation.historySize
<number> Maximum number of history lines retained. To disable the history set this value to0
. This option makes sense only ifterminal
is set totrue
by the user or by an internaloutput
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
exceedscrlfDelay
milliseconds, both\r
and\n
will be treated as separate end-of-line input.crlfDelay
will be coerced to a number no less than100
. It can be set toInfinity
, 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> Iftrue
, 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 durationreadline
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.
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:
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]
.
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:
function completer(linePartial, callback) {
callback(null, [['123'], linePartial]);
}
readline.cursorTo(stream, x[, y][, callback])[src]#
stream
<stream.Writable>x
<number>y
<number>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The readline.cursorTo()
method moves cursor to the specified position in a
given TTY stream
.
readline.emitKeypressEvents(stream[, interface])[src]#
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.
readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
process.stdin.setRawMode(true);
readline.moveCursor(stream, dx, dy[, callback])[src]#
stream
<stream.Writable>dx
<number>dy
<number>callback
<Function> Invoked once the operation completes.- Returns: <boolean>
false
ifstream
wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
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:
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:
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:
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:
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);
}
})();