en.javascript.info/1-js/02-first-steps/08-operators/4-fix-prompt/solution.md

The reason is that prompt returns user input as a string.

So variables have values "1" and "2" respectively.

let a = "1"; // prompt("First number?", 1);
let b = "2"; // prompt("Second number?", 2);

alert(a + b); // 12

What we should to is to convert strings to numbers before +. For example, using Number() or prepending them with +.

For example, right before prompt:

let a = +prompt("First number?", 1);
let b = +prompt("Second number?", 2);

alert(a + b); // 3

Or in the alert:

let a = prompt("First number?", 1);
let b = prompt("Second number?", 2);

alert(+a + +b); // 3

Using both unary and binary + in the latest code. Looks funny, doesn't it?