bob(1)(2)(3)(4)...(n) + '' == 1 + 2 + 3 + 4 + ... + n

bob.js

function bob(val) {
  if (this === window) {
    return (new bob(val)).factorInc();
  }
  else {
    this.v = val;
  }
}

bob.prototype.inc = function(v) {
  this.v += v;
  return this.factorInc();
}

bob.prototype.toString = function() {
  return this.v;
}

bob.prototype.factorInc = function() {
  var inc = this.inc.bind(this);
  inc.toString = this.toString.bind(this);
  return inc;
}