+!! operator in an if statement

!! converts a value to a boolean (true or false). + then converts that boolean to a number, either 1 for true or 0 for false.

> +true
1
> +false
0

Personally I find it clearer to write something like this, when dealing with two booleans:

if (!config.template == !config.templateUrl) {
  throw ...
}

Code clarity and readability be damned, apparently.