javascript: How to use the Object.assign function with arrays that contain null elements?

I discovered that the assignment function does not work when the initial matrices contain null elements. Two examples below clearly show this:

var x = ["title","t1"];
var y = []; Y[3]= "t3";

console.log (x);
console.log (y);

Object.assign (x, y);

console.log (x);
console.log (y);
var x = ["title","t1"];
var y = [null,null,null,"t3"];

console.log (x);
console.log (y);

Object.assign (x, y);

console.log (x);
console.log (y);

Is it possible to make the Object.assign function work with arrays that contain null elements like these?

var y = [null,null,null,"t3"];