how to work with "process.stdin.on"?

Question

I'm trying to understand process.stdin.

For example - I need to show array elements in console. And i should allow user choose which element will be shown.

I have code:

var arr = ['elem1','elem2','elem3','elem4','elem5'],
    lastIndx = arr.length-1;

showArrElem();

function showArrElem () {

  console.log('press number from 0 to ' + lastIndx +', or "q" to quit');

  process.stdin.on('readable', function (key) {
        var key = process.stdin.read();
        if (!process.stdin.isRaw) {
          process.stdin.setRawMode( true );
        } else {
          var i = String(key);
          if (i == 'q') {
            process.exit(0);
          } else {
            console.log('you press ' +i); // null
            console.log('e: ' +arr[i]);
            showArrElem();
          };
        };  
  });

};

Why the "i" is null when i type number a second time? How to use "process.stdin.on" correctly?

Answer 1

You're attaching a readable listener on process.stdin after every input character, which is causing process.stdin.read() to be invoked more than one time for each character. stream.Readable.read(), which process.stdin is an instance of, returns null if there's no data in the input buffer. To work around this, attach the listener once.

process.stdin.setRawMode(true);
process.stdin.on('readable', function () {
  var key = String(process.stdin.read());
  showArrEl(key);
});

function showArrEl (key) {
  console.log(arr[key]);
}

Alternatively, you can attach a one-time listener using process.stdin.once('readable', ...).