Why d (variable)is treated as string

Question

I don't know why it's happening d is treated as string
https://code.sololearn.com/WruS83awNHHw/?ref=app

Answer 1

That's because prompt accepts strings only.
Do like the following:
var y = Number(prompt("Enter year"));

Answer 2

use parseInt to convert it to integer.

Answer 3

thx a lot

Answer 4

how to split a numeric data input
like I have to use 3 of 5340

Answer 5

Covert it to a string, then get the index of your number and catch it with substr.
For example,
var x = 5340
alert(x.toString().substr(1,1))