I’d argue that

function add (arr) {
  return arr.reduce((prev, current) => prev + current, 0)
}

While more declarative than looping through the array, is still more imperative than something like arr.sum. Here’s how I define the difference:

• Imperative Code: Describe strategy, hope computer finds correct result.
• Declarative Code: Describe result, hope computer finds optimum strategy.

Note this doesn’t mean that your code is side-effect free. As an extreme example, consider something like next(foo) in Python where foo is an iterator.

tl;dr everything is fractals