Node.js - promisify readline

Question

As the title states, I was wondering if it is possible to use promisify (https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original) for readline in node.js? I was only able to do it like this:

let data = [];
const parse = () => {
    return new Promise((resolve, reject) => {

        const rl = readline.createInterface({
            input: fs.createReadStream(path)
        });

        rl.on('line', (line) => {
            data.push(line);
        });

        rl.on('close', () => {
            resolve(data);
        });
    });
};

You can only promisify error first callbacks, and with promisify.custom other types of callbacks that does not follow the pattern. For streams and event emitters you need to implement your own logic, it does share a common interface (with on close, on finish ) but the usecases are very different. — Alexandru Olaru
– Alexandru Olaru, Commented Oct 24, 2017 at 10:35
Yeah, that's what I suspected. Is my implementation adequate? — uglycode
– uglycode, Commented Oct 24, 2017 at 10:48
The problem with your implementation is that you added the data in a higher scope than your promise, it will accumulate data for each parse use, if you use parse promise 2 times, the second use will have the first values appended and then the second values also. A better approach is to set the let data in the promise in this way for each use you will store only the new data. — Alexandru Olaru
– Alexandru Olaru, Commented Oct 24, 2017 at 10:54

Jesús López · Accepted Answer · 2022-03-21 18:57:53Z

17

Here you have a simple way to do it that doesn't use promisify:

const readline = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout
});

function question(query) {
    return new Promise(resolve => {
        readline.question(query, resolve);
    });
}

async function main() {
    const name = await question('Whats your name? ');
    console.log(`hello ${name}!`);
    readline.close();
}

main();

edited Mar 21, 2022 at 18:57

answered Dec 26, 2020 at 11:33

Jesús López

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1 Comment

masterxilo Over a year ago

starting in node 17, you can simply do readline = require('readline').promises.createInterface(...) and then just invoke await readline.question('...') directly, see nodejs.org/api/readline.html#class-readlinepromisesinterface

romellem · Accepted Answer · 2020-07-01 14:48:39Z

7

Here is an example of how I promisify readline.question:

const rl = require('readline');
const { promisify } = require('util');
const readline = rl.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Prepare readline.question for promisification
readline.question[promisify.custom] = (question) => {
  return new Promise((resolve) => {
    readline.question(question, resolve);
  });
};

// Usage example:
(async () => {
  const answer = await promisify(readline.question)('What is your name? ');
  readline.close();
  console.log('Hi %s!', answer);
})();

Node (v8) documentation for custom promisified functions: https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_custom_promisified_functions

edited Jul 1, 2020 at 14:48

romellem

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answered Feb 19, 2019 at 8:10

Josef Engelfrost

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Comments

John Michael Villegas · Accepted Answer · 2017-10-24 10:34:51Z

0

try to use bluebird which create http://bluebirdjs.com/docs/api/promise.promisifyall.html

but if the code works. then I think you don't need to promisify that since you already return it as promise.

answered Oct 24, 2017 at 10:34

John Michael Villegas

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Comments

fregante · Accepted Answer · 2023-07-04 10:44:23Z

0

Starting in Node 17, it's already promisified:

import { stdin, stdout } from 'node:process';
import * as readline from 'node:readline/promises';

const rl = readline.createInterface({ input: stdin, output: stdout });
const answer = await rl.question('What is your name? ');
console.log('Thank you', answer);
rl.close();

See the documentation:

https://nodejs.org/api/readline.html#rlquestionquery-options

answered Jul 4, 2023 at 10:44

fregante

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