Node JS with Express API returning null instead undefined in array items

I have an API endpoint in my Node JS express app as below:

app.get('/src/static', (req, res) => {
  res.send({ hello: [1, undefined, 2, 3] });
});

Which is returning the response as below when I call that endpoint:

{"hello":[1,null,2,3]}

In the response above, It should return the undefined in the array. but, It is returning the null.

Additional details:

Node JS: 14
"express": "^4.17.1"

What could be the problem?