1. Introduction
The readline module allows developers to read input line by line from streams. It can be loaded with require('node:readline') or the Promise-based version require('node:readline/promises').
2. Creating an Interface
const readline = require('node:readline');
const { stdin: input, stdout: output } = require('node:process');
const rl = readline.createInterface({ input, output });
3. Key Events
- line: Triggered when a line of input is received.
- close: Triggered when the interface is closed.
- pause / resume: Manage input stream state.
- SIGINT: Triggered by Ctrl+C.
- SIGTSTP / SIGCONT: Handle backgrounding and resuming processes.
- history: Triggered when the input history changes.
4. Core Methods
rl.question(query, callback): Displays a query and captures user input.rl.prompt(): Displays the prompt and waits for input.rl.setPrompt(prompt): Sets the prompt text.rl.getPrompt(): Returns the current prompt.rl.write(data[, key]): Writes data or simulates key input.rl.close(): Closes the interface.
5. Async Iteration
The readline interface supports for await...of loops for asynchronous line-by-line processing:
for await (const line of rl) {
console.log(`Received: ${line}`);
}
6. Example Use Cases
- Simple CLI: Build interactive command-line tools with custom commands.
- File Processing: Read files line-by-line using
fs.createReadStream()with readline.
7. TTY Keybindings
The readline module supports keybindings such as Ctrl+C, Ctrl+D, Ctrl+U, and Ctrl+K for managing input in terminal sessions.
Conclusion
The readline module is a powerful tool for building interactive applications in Node.js. With support for both Promise-based and Callback-based APIs, rich event handling, and file processing capabilities, it is essential for developing CLIs and data-processing utilities.