JS weird += result [duplicate]

I used this function to add 5 to a number in a p element:

function add5() {lifeTotal.textContent += 5;}

I have a similar function that subtracts 5 to that same number:

function minus5() {lifeTotal.textContent -= 5;}

Now my problem is that while the subtracting function works as expected, the adding one just puts 5 at the right side of the number. So if the number was 10, it would return 105, as if both numbers were actually strings and not numbers… while the subtracting function works normally and treats numbers as numbers…

It turns me crazy. I just learnt this += thing and I really don’t think I used it wrong…

So my solution is to instead use ++ five times, like so:

function add5() {lifeTotal.textContent ++; lifeTotal.textContent ++; lifeTotal.textContent ++; lifeTotal.textContent ++; lifeTotal.textContent ++;}

It works, but it seems to me this is not the shortest way to write this.

The app I am writing is adding or subtracting by calling the corresponding function with an onclick=”function()” in html button tags.

What is happening? What is the shortest way to write this?

Thanks!