What is the output of c? Its output is NaN. HOW???(JS)

Question

var x=4;
var y=5;
var d=4+z;
var z=y-x;
var c=x+z+d+y;
alert(c);

Answer 1

NAN means Not-a-number.
When you try to perform a mathematical operation(+,-,×,/) with undefined(here z) it cannot be coerced to a different type or a number type in this case so the result of the operation cannot be evaluated and returns NaN.

Answer 2

Because, you assigned variable z before declaring it.
Do it like this

var x=4;
var y=5;
var z=y-x;
var d=4+z;
var c=x+z+d+y;
alert(c);

Answer 3

You have d=4+z here z in uninitialized. First take var z= ..
Then var d=..

edit: Bikesh Shrestha

var x=4;
var y=5;
var z=y-x;
var d=4+z;
var c=x+z+d+y;

alert(c);

Answer 4

No no the question asked was like this only. So i want to know how it becomes NaN.
Thank you so much for helping me out in this JS question.