> I stopped at the first example (true + false).

So you never did something like this:

  x = performCalculation(getParam1())
  y = performCalculation(getParam2())
  sum = x + y

only to discover that "performCalculation()" sometimes returns a boolean instead of a number? JS is a dynamically typed language after all and functions like

  function f(x) {
    if (x >= 0) {
      return Math.sqrt(x)
    } else {
      return false
    }
  }

are perfectly valid. If you use a 3rd party library that uses return values like this, you might run into such case without realising it.

Sure, you won't explicitly write "true + false", but "f(x) + g(x)" is not uncommon and might indeed evaluate to "true + false".