0.1 + 0.2 = 0.3

0.3.js

function big(n, m) {
  var a1 = f(n), a2 = f(m);
  var overflow = d(a1[1], a2[1]);
  arr = [a1[0] + a2[0] + overflow[0], overflow[1]];
  return +arr.join('.');
}

function f(n) {
  var tmp = ('' + n).split('.');
  if(tmp.length === 1) tmp[1] = '0';
  tmp[0] = +tmp[0];
  return tmp;
}

function d(a, b) {
  var l = Math.max(a.length, b.length);
  var r = +a + (+b);
  var base = Math.pow(10, l);
  return [r >= base ? 1 : 0, r % base];
}

var normal = 0, modified = 0;

for(var i = 0; i < 1e7; i++) {
  normal += 0.1 + 0.2;
  modified = big(modified, big(0.1, 0.2));
}

console.log(normal, modified);