How can i make the user to type the variables in Java script?

Question

if I decide a variable to make the user choose it the calc program must be done by me

Answer 1

vr = prompt("Variable Name");
eval(vr+"='value';");

Answer 2

if u mean ask the input from user and set that input as a variable this is what u should do
var x = prompt("Enter input")
[hope I am not wrong cause I have not tries it yet]