How to get synchronous readline, or "simulate" it using async, in nodejs?

Question

I am wondering if there is a simple way to get "synchronous" readline or at least get the appearance of synchronous I/O in node.js

I use something like this but it is quite awkward

var readline = require('readline');
var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  terminal: false
});

var i = 0;
var s1 = '';
var s2 = '';

rl.on('line', function(line){
    if(i==0) { s1 = line; }
    else if(i==1) { s2 = line; }
    i++;
})

rl.on('close', function() {
    //do something with lines
})'

Instead of this I would prefer if it were as easy as something like

var s1 = getline(); // or "await getline()?"
var s2 = getline(); // or "await getline()?"

Helpful conditions:

(a) Prefer not using external modules or /dev/stdio filehandle, I am submitting code to a code submission website and these do not work there

(b) Can use async/await or generators

(c) Should be line based

(d) Should not require reading entire stdin into memory before processing

Answer 1

Just in case someone stumbles upon here in future

Node 11.7 added support for this using async await

const readline = require('readline');
//const fileStream = fs.createReadStream('input.txt');

const rl = readline.createInterface({
  input: process.stdin, //or fileStream 
  output: process.stdout
});

for await (const line of rl) {
  console.log(line)
}

Remember to wrap it in async function(){} otherwise you will get a reserved_keyword_error.

const start = async () =>{
    for await (const line of rl) {
        console.log(line)
    }
}
start()

To read an individual line, you can use the async iterator manually

const it = rl[Symbol.asyncIterator]();
const line1 = await it.next();

Answer 2

Like readline module, there is another module called readline-sync, which takes synchronous input.

Example:

const reader = require("readline-sync"); //npm install readline-sync
let username = reader.question("Username: ");
const password = reader.question("Password: ",{ hideEchoBack: true });
if (username == "admin" && password == "foobar") {
    console.log("Welcome!")
}

Answer 3

You can just wrap it in a promise -

const answer = await new Promise(resolve => {
  rl.question("What is your name? ", resolve)
})
console.log(answer)

Answer 4

I think this is what you want :

const readline = require('readline');

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

const getLine = (function () {
    const getLineGen = (async function* () {
        for await (const line of rl) {
            yield line;
        }
    })();
    return async () => ((await getLineGen.next()).value);
})();

const main = async () => {
    let a = Number(await getLine());
    let b = Number(await getLine());
    console.log(a+b);
    process.exit(0);
};

main();

Note: this answer use experimental features and need Node v11.7