Read file stream line by line using generators

0-Description.md

A follow-up to http://yarfrontend.ru/talks/es6-generators/.

Goals:

1. Iterate over readable stream, line by line, using generators.
2. Write async code in synchronous way but hide implementation details as much as possible

Async wrapper live demo: http://jsfiddle.net/Olegas/v8Lujpr8/4/

Async wrapper

// You need some block element with id = 'log' to see output

asyncWrap(function *() {

    var currentTime;
    
    log('Lets wait 5 sec');
    
    currentTime = yield promiseFromTimer(5000);
    
    log('Current time is ' + currentTime + '. Now lets wait 10 sec');
    
    try {
        currentTime = yield exceptionFromTimer(10000);
        // Will never happen
        log('Current time is ' + currentTime + '. Ok then, lets stop');
    } catch(e) {
        log('Got exception: ' + e + ' at ' + (new Date()));
    }
    
    var fourtytwo = yield plusTwo(40);
    
    log('And... the meaning of life is ' + fourtytwo);
    
});

function asyncWrap(g) {

    function step(v) {
        var val = (v instanceof Error ? it.throw(v) : it.next(v));
        if (val.done) {
            return;
        }
        val = val.value;
        if (val instanceof Promise) {
            val.then(step, step);
        } else {
           step(val);
        }
    }

    var it = g();
    step();
}

function plusTwo(n) {
    return n + 2;
}

function promiseFromTimer(delay) {
    return new Promise(function(fulfil) {
        setTimeout(fulfil.bind(null, new Date()) , delay);
    });
}

function exceptionFromTimer(delay) {
    return new Promise(function(fulfil, reject) {
        setTimeout(reject.bind(null, new Error('Oh!')), delay);
    });
}

function log(s) {
    var p = document.createElement('p');
    p.appendChild(document.createTextNode(s));
    document.getElementById('log').appendChild(p);
}


    

fileReader.js

function readStreamByLine(stream, handler) {

   var res = '', completed = false;

   stream.pause();

   stream.on('data', function(chunk) {
      // Assume we have only ascii data, no UTF
      res += chunk.toString();
      check();
   });

   stream.on('end', function() {
      completed = true;
      check();
   });

   function check() {
      var
         nlIdx = res.indexOf('\n'),
         line;
      if (nlIdx !== -1) {
         line = res.substr(0, nlIdx);
         res = res.substr(nlIdx + 1);
         stream.pause();
         it.next(line);
      } else {
         if (completed) {
            res && it.next(res);
            it.next(false);
         } else {
            stream.resume();
         }
      }
   }

   var it = handler(function() {
      setImmediate(check);
   });
   it.next();
}


var strm = require('fs').createReadStream(__filename);
readStreamByLine(strm, function *gen(nextPlease) {

   while(true) {
      var line = yield nextPlease();
      if (line === false) {
         return;
      }
      console.log('Got line: ' + line);
   }
});