Table of Contents

• Readline

  • Class: Interface

    • Event: 'close'
    • Event: 'line'
    • Event: 'pause'
    • Event: 'resume'
    • Event: 'SIGCONT'
    • Event: 'SIGINT'
    • Event: 'SIGTSTP'
    • rl.close()
    • rl.pause()
    • rl.prompt([preserveCursor])
    • rl.question(query, callback)
    • rl.resume()
    • rl.setPrompt(prompt)
    • rl.write(data[, key])
    • rl[Symbol.asyncIterator]()
    • rl.line
    • rl.cursor
    • rl.getCursorPos()
  • readline.clearLine(stream, dir[, callback])
  • readline.clearScreenDown(stream[, callback])

  • readline.createInterface(options)

    • Use of the completer Function
  • readline.cursorTo(stream, x[, y][, callback])
  • readline.emitKeypressEvents(stream[, interface])
  • readline.moveCursor(stream, dx, dy[, callback])
  • Example: Tiny CLI
  • Example: Read File Stream Line-by-Line

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 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'#

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> 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)#

• 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:

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])#

• 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.

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`.
  }
}

rl.line#

• <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:

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#

• <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()#

• 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])#

• 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])#

• 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)#

• 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.

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])#

• 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])#

• 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])#

• 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:

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);
  }
})();