var body = document.getElementsByTagName('body')[0],

    pb = document.getElementById('parentBox'),

    cb = document.getElementById('childBox'),

    gcb = document.getElementById('grandchildBox'),

    ob = document.getElementById('otherBox');

​

console.log(body.contains(pb)); // true

console.log(body.contains(gcb)); // true

console.log(pb.contains(cb)); // true

console.log(pb.contains(gcb)); // true

console.log(cb.contains(gcb)); // true

console.log(cb.contains(pb)); // false

console.log(body.contains(ob)); // true

console.log(pb.contains(ob)); // false