var ref = new Reference();
var arr = [1, 2, ref];
ref.overwrite(3);
console.log(arr); // output: [1,2,3]

var arr = [1, 2, null];
var ref = {object: arr, index: 2};
ref.object[ref.index] = 3;
console.log(arr); // output: [1, 2, 3]