serially read lines via asynchronous readline interface using semaphore with capacity 1

serial-readline.js

// create a file named input containing 1, 2, 3, 4 on separate lines
// cat input | node serial-readline.js
// output (shows bot stdin and stdout):
//      1
//      start handler
//      end handler
//      2
//      3
//      4processing: 1
//      start handler
//      end handler
//      processing: 2
//      start handler
//      end handler
//      processing: 3
//      start handler
//      end handler
//      processing: 4

const readline = require('readline');
var sem = require('semaphore')(1);

const someThingAsync = async data => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('processing: ' + data);
            resolve(data);
            sem.leave();
        }, Math.random() * 500);
    });
};

const handler = data => {
    sem.take(function () {
        // notice that this function is NOT async
        console.log('start handler');
        someThingAsync(data);
        console.log('end handler');
    });
};

function run() {
    this.io = readline.createInterface(process.stdin, process.stdout);
    this.io.on('line', handler);
}

run();