Why is it necessary to create a new variable every time when working with methods?

Question

JavaScript

Good morning, tell me why you can not write like this?

var str = "car";
str.split().splice(1,0, "is black");

or so

var str = "car";
var res = str.split().splice(1,0, "is black");

And requires precisely so, in two lines.

var str = "car";
var res = str.split();
res.splice(1,0, "is black");

But even continuing such a chain, we cannot write

var str = "car";
var res = str.split();
res.splice(1,0, "is black");
res.join("");

And forced to create a new variable forres.join("")

Answer 1

splice modifies an array, but returns not the modified array, but what was removed. You have 0 elements removed, so the return value is an empty array.
It's not clear why you use splice at all in this example.
You can throw everything away and write simply:
"car is black"