{ "type": "module", "source": "doc/api/readline.md", "modules": [ { "textRaw": "Readline", "name": "readline", "introduced_in": "v0.10.0", "stability": 2, "stabilityText": "Stable", "desc": "
The readline
module provides an interface for reading data from a Readable\nstream (such as process.stdin
) one line at a time. It can be accessed using:
const readline = require('readline');\n
\nThe following simple example illustrates the basic use of the readline
module.
const readline = require('readline');\n\nconst rl = readline.createInterface({\n input: process.stdin,\n output: process.stdout\n});\n\nrl.question('What do you think of Node.js? ', (answer) => {\n // TODO: Log the answer in a database\n console.log(`Thank you for your valuable feedback: ${answer}`);\n\n rl.close();\n});\n
\nOnce this code is invoked, the Node.js application will not terminate until the\nreadline.Interface
is closed because the interface waits for data to be\nreceived on the input
stream.
Instances of the readline.Interface
class are constructed using the\nreadline.createInterface()
method. Every instance is associated with a\nsingle input
Readable stream and a single output
Writable stream.\nThe output
stream is used to print prompts for user input that arrives on,\nand is read from, the input
stream.
The 'close'
event is emitted when one of the following occur:
rl.close()
method is called and the readline.Interface
instance has\nrelinquished control over the input
and output
streams;input
stream receives its 'end'
event;input
stream receives <ctrl>-D
to signal end-of-transmission (EOT);input
stream receives <ctrl>-C
to signal SIGINT
and there is no\n'SIGINT'
event listener registered on the readline.Interface
instance.The listener function is called without passing any arguments.
\nThe readline.Interface
instance is finished once the 'close'
event is\nemitted.
The 'line'
event is emitted whenever the input
stream receives an\nend-of-line input (\\n
, \\r
, or \\r\\n
). This usually occurs when the user\npresses the <Enter>
, or <Return>
keys.
The listener function is called with a string containing the single line of\nreceived input.
\nrl.on('line', (input) => {\n console.log(`Received: ${input}`);\n});\n
"
},
{
"textRaw": "Event: 'pause'",
"type": "event",
"name": "pause",
"meta": {
"added": [
"v0.7.5"
],
"changes": []
},
"params": [],
"desc": "The 'pause'
event is emitted when one of the following occur:
input
stream is paused.input
stream is not paused and receives the 'SIGCONT'
event. (See\nevents 'SIGTSTP'
and 'SIGCONT'
.)The listener function is called without passing any arguments.
\nrl.on('pause', () => {\n console.log('Readline paused.');\n});\n
"
},
{
"textRaw": "Event: 'resume'",
"type": "event",
"name": "resume",
"meta": {
"added": [
"v0.7.5"
],
"changes": []
},
"params": [],
"desc": "The 'resume'
event is emitted whenever the input
stream is resumed.
The listener function is called without passing any arguments.
\nrl.on('resume', () => {\n console.log('Readline resumed.');\n});\n
"
},
{
"textRaw": "Event: 'SIGCONT'",
"type": "event",
"name": "SIGCONT",
"meta": {
"added": [
"v0.7.5"
],
"changes": []
},
"params": [],
"desc": "The 'SIGCONT'
event is emitted when a Node.js process previously moved into\nthe background using <ctrl>-Z
(i.e. SIGTSTP
) is then brought back to the\nforeground using fg(1p)
.
If the input
stream was paused before the SIGTSTP
request, this event will\nnot be emitted.
The listener function is invoked without passing any arguments.
\nrl.on('SIGCONT', () => {\n // `prompt` will automatically resume the stream\n rl.prompt();\n});\n
\nThe 'SIGCONT'
event is not supported on Windows.
The 'SIGINT'
event is emitted whenever the input
stream receives a\n<ctrl>-C
input, known typically as SIGINT
. If there are no 'SIGINT'
event\nlisteners registered when the input
stream receives a SIGINT
, the 'pause'
\nevent will be emitted.
The listener function is invoked without passing any arguments.
\nrl.on('SIGINT', () => {\n rl.question('Are you sure you want to exit? ', (answer) => {\n if (answer.match(/^y(es)?$/i)) rl.pause();\n });\n});\n
"
},
{
"textRaw": "Event: 'SIGTSTP'",
"type": "event",
"name": "SIGTSTP",
"meta": {
"added": [
"v0.7.5"
],
"changes": []
},
"params": [],
"desc": "The 'SIGTSTP'
event is emitted when the input
stream receives a <ctrl>-Z
\ninput, typically known as SIGTSTP
. If there are no 'SIGTSTP'
event listeners\nregistered when the input
stream receives a SIGTSTP
, the Node.js process\nwill be sent to the background.
When the program is resumed using fg(1p)
, the 'pause'
and 'SIGCONT'
events\nwill be emitted. These can be used to resume the input
stream.
The 'pause'
and 'SIGCONT'
events will not be emitted if the input
was\npaused before the process was sent to the background.
The listener function is invoked without passing any arguments.
\nrl.on('SIGTSTP', () => {\n // This will override SIGTSTP and prevent the program from going to the\n // background.\n console.log('Caught SIGTSTP.');\n});\n
\nThe 'SIGTSTP'
event is not supported on Windows.
The rl.close()
method closes the readline.Interface
instance and\nrelinquishes control over the input
and output
streams. When called,\nthe 'close'
event will be emitted.
Calling rl.close()
does not immediately stop other events (including 'line'
)\nfrom being emitted by the readline.Interface
instance.
The rl.pause()
method pauses the input
stream, allowing it to be resumed\nlater if necessary.
Calling rl.pause()
does not immediately pause other events (including\n'line'
) from being emitted by the readline.Interface
instance.
The rl.prompt()
method writes the readline.Interface
instances configured\nprompt
to a new line in output
in order to provide a user with a new\nlocation at which to provide input.
When called, rl.prompt()
will resume the input
stream if it has been\npaused.
If the readline.Interface
was created with output
set to null
or\nundefined
the prompt is not written.
The rl.question()
method displays the query
by writing it to the output
,\nwaits for user input to be provided on input
, then invokes the callback
\nfunction passing the provided input as the first argument.
When called, rl.question()
will resume the input
stream if it has been\npaused.
If the readline.Interface
was created with output
set to null
or\nundefined
the query
is not written.
Example usage:
\nrl.question('What is your favorite food? ', (answer) => {\n console.log(`Oh, so your favorite food is ${answer}`);\n});\n
\nThe callback
function passed to rl.question()
does not follow the typical\npattern of accepting an Error
object or null
as the first argument.\nThe callback
is called with the provided answer as the only argument.
The rl.resume()
method resumes the input
stream if it has been paused.
The rl.setPrompt()
method sets the prompt that will be written to output
\nwhenever rl.prompt()
is called.
The rl.write()
method will write either data
or a key sequence identified\nby key
to the output
. The key
argument is supported only if output
is\na TTY text terminal.
If key
is specified, data
is ignored.
When called, rl.write()
will resume the input
stream if it has been\npaused.
If the readline.Interface
was created with output
set to null
or\nundefined
the data
and key
are not written.
rl.write('Delete this!');\n// Simulate Ctrl+u to delete the line written previously\nrl.write(null, { ctrl: true, name: 'u' });\n
\nThe rl.write()
method will write the data to the readline
Interface
's\ninput
as if it were provided by the user.
Create an AsyncIterator
object that iterates through each line in the input\nstream as a string. This method allows asynchronous iteration of\nreadline.Interface
objects through for
-await
-of
loops.
Errors in the input stream are not forwarded.
\nIf the loop is terminated with break
, throw
, or return
,\nrl.close()
will be called. In other words, iterating over a\nreadline.Interface
will always consume the input stream fully.
A caveat with using this experimental API is that the performance is\ncurrently not on par with the traditional 'line'
event API, and thus it is\nnot recommended for performance-sensitive applications. We expect this\nsituation to improve in the future.
async function processLineByLine() {\n const rl = readline.createInterface({\n // ...\n });\n\n for await (const line of rl) {\n // Each line in the readline input will be successively available here as\n // `line`.\n }\n}\n
"
}
]
}
],
"methods": [
{
"textRaw": "readline.clearLine(stream, dir)",
"type": "method",
"name": "clearLine",
"meta": {
"added": [
"v0.7.7"
],
"changes": []
},
"signatures": [
{
"params": [
{
"textRaw": "`stream` {stream.Writable}",
"name": "stream",
"type": "stream.Writable"
},
{
"textRaw": "`dir` {number}",
"name": "dir",
"type": "number",
"options": [
{
"textRaw": "`-1` - to the left from cursor",
"name": "-1",
"desc": "to the left from cursor"
},
{
"textRaw": "`1` - to the right from cursor",
"name": "1",
"desc": "to the right from cursor"
},
{
"textRaw": "`0` - the entire line",
"name": "0",
"desc": "the entire line"
}
]
}
]
}
],
"desc": "The readline.clearLine()
method clears current line of given TTY stream\nin a specified direction identified by dir
.
The readline.clearScreenDown()
method clears the given TTY stream from\nthe current position of the cursor down.
The readline.createInterface()
method creates a new readline.Interface
\ninstance.
const readline = require('readline');\nconst rl = readline.createInterface({\n input: process.stdin,\n output: process.stdout\n});\n
\nOnce the readline.Interface
instance is created, the most common case is to\nlisten for the 'line'
event:
rl.on('line', (line) => {\n console.log(`Received: ${line}`);\n});\n
\nIf terminal
is true
for this instance then the output
stream will get\nthe best compatibility if it defines an output.columns
property and emits\na 'resize'
event on the output
if or when the columns ever change\n(process.stdout
does this automatically when it is a TTY).
The completer
function takes the current line entered by the user\nas an argument, and returns an Array
with 2 entries:
Array
with matching entries for the completion.For instance: [[substr1, substr2, ...], originalsubstring]
.
function completer(line) {\n const completions = '.help .error .exit .quit .q'.split(' ');\n const hits = completions.filter((c) => c.startsWith(line));\n // Show all completions if none found\n return [hits.length ? hits : completions, line];\n}\n
\nThe completer
function can be called asynchronously if it accepts two\narguments:
function completer(linePartial, callback) {\n callback(null, [['123'], linePartial]);\n}\n
",
"type": "module",
"displayName": "Use of the `completer` Function"
}
]
},
{
"textRaw": "readline.cursorTo(stream, x, y)",
"type": "method",
"name": "cursorTo",
"meta": {
"added": [
"v0.7.7"
],
"changes": []
},
"signatures": [
{
"params": [
{
"textRaw": "`stream` {stream.Writable}",
"name": "stream",
"type": "stream.Writable"
},
{
"textRaw": "`x` {number}",
"name": "x",
"type": "number"
},
{
"textRaw": "`y` {number}",
"name": "y",
"type": "number"
}
]
}
],
"desc": "The readline.cursorTo()
method moves cursor to the specified position in a\ngiven TTY stream
.
The readline.emitKeypressEvents()
method causes the given Readable\nstream to begin emitting 'keypress'
events corresponding to received input.
Optionally, interface
specifies a readline.Interface
instance for which\nautocompletion 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\ninput
is a terminal. Closing the readline
instance does not stop\nthe input
from emitting 'keypress'
events.
readline.emitKeypressEvents(process.stdin);\nif (process.stdin.isTTY)\n process.stdin.setRawMode(true);\n
"
},
{
"textRaw": "readline.moveCursor(stream, dx, dy)",
"type": "method",
"name": "moveCursor",
"meta": {
"added": [
"v0.7.7"
],
"changes": []
},
"signatures": [
{
"params": [
{
"textRaw": "`stream` {stream.Writable}",
"name": "stream",
"type": "stream.Writable"
},
{
"textRaw": "`dx` {number}",
"name": "dx",
"type": "number"
},
{
"textRaw": "`dy` {number}",
"name": "dy",
"type": "number"
}
]
}
],
"desc": "The readline.moveCursor()
method moves the cursor relative to its current\nposition in a given TTY stream
.
The following example illustrates the use of readline.Interface
class to\nimplement a small command-line interface:
const readline = require('readline');\nconst rl = readline.createInterface({\n input: process.stdin,\n output: process.stdout,\n prompt: 'OHAI> '\n});\n\nrl.prompt();\n\nrl.on('line', (line) => {\n switch (line.trim()) {\n case 'hello':\n console.log('world!');\n break;\n default:\n console.log(`Say what? I might have heard '${line.trim()}'`);\n break;\n }\n rl.prompt();\n}).on('close', () => {\n console.log('Have a great day!');\n process.exit(0);\n});\n
\nA common use case for readline
is to consume an input file one line at a\ntime. The easiest way to do so is leveraging the fs.ReadStream
API as\nwell as a for
-await
-of
loop:
const fs = require('fs');\nconst readline = require('readline');\n\nasync function processLineByLine() {\n const fileStream = fs.createReadStream('input.txt');\n\n const rl = readline.createInterface({\n input: fileStream,\n crlfDelay: Infinity\n });\n // Note: we use the crlfDelay option to recognize all instances of CR LF\n // ('\\r\\n') in input.txt as a single line break.\n\n for await (const line of rl) {\n // Each line in input.txt will be successively available here as `line`.\n console.log(`Line from file: ${line}`);\n }\n}\n\nprocessLineByLine();\n
\nAlternatively, one could use the 'line'
event:
const fs = require('fs');\nconst readline = require('readline');\n\nconst rl = readline.createInterface({\n input: fs.createReadStream('sample.txt'),\n crlfDelay: Infinity\n});\n\nrl.on('line', (line) => {\n console.log(`Line from file: ${line}`);\n});\n
\nCurrently, for
-await
-of
loop can be a bit slower. If async
/ await
\nflow and speed are both essential, a mixed approach can be applied:
const { once } = require('events');\nconst { createReadStream } = require('fs');\nconst { createInterface } = require('readline');\n\n(async function processLineByLine() {\n try {\n const rl = createInterface({\n input: createReadStream('big-file.txt'),\n crlfDelay: Infinity\n });\n\n rl.on('line', (line) => {\n // Process the line.\n });\n\n await once(rl, 'close');\n\n console.log('File processed.');\n } catch (err) {\n console.error(err);\n }\n})();\n
"
}
],
"type": "module",
"displayName": "Readline"
}
]
}