Sum of two numbers with prompt

The function prompt returns a string and + is (unwisely, perhaps) used for both string concatenation and number addition.

You do not “specify types” in JavaScript but you can do string to number conversion at run time. There are many ways to so this. The simplest is:

var a = +prompt("Enter first number");
var b = +prompt("Enter second number");
alert(a + b);

but you can also do

var a = Number(prompt("Enter first number"));
var b = Number(prompt("Enter second number"));
alert(a + b);

(Avoid parseInt because it only handles the leading characters and will not add numbers like 4.5 and 2.6.)