http://www.sitepoint.com/javascript-truthy-falsy/

function assert(condition, message) {
  if (!condition) {
    throw message || "Assertion failed";
  }
}

// XXX == false
//
// falsy
assert(!(false));
assert(!(0));
assert(!(""));
assert(!(null));
assert(!(undefined));
assert(!(NaN));

// truthy
assert(!!("0"));
assert(!!("false"));
// empty functions
// empty array
// empty object