You cannot do multiple inheritance in JavaScript. You can do deeper inheritance just by going further:
var Parent = function() {};
var Child = function() {};
var InnerChild = function() {};
And to show that it works:
Parent.prototype.x = 100;
Child.prototype = new Parent();
Child.prototype.y = 200;
InnerChild.prototype = new Child();
InnerChild.prototype.z = 300;
var ic = new InnerChild();
console.log(ic.x);
console.log(ic.y);
console.log(ic.z);
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