The forEach() method needs two arguments. First is the callback function and the second is the element which the this keyword can refer to the callback function.
In the totalArray.forEach(adder());, you are executing the adder function. Since adder is not returning anything from its body, you are getting undefined.
As a result, your code is effectively calling the forEach() with undefined as the argument.
For your code to run correctly, just omit the parenthesis while calling the function.