Reorder your initial array and what result would you get? This is great for composability and shows the strength in isolating atomic behaviour into individual functions. It totals up, and then on the last element of the array, it divides by the length of the array passed in. Here's an implementation of reduce that averages an array of numbers. But here's some more interesting cases you might come across one day. I have little to add on teaching you the common use cases of reduce. Thankfully, the internet is littered with blog posts from developers of all statures giving their two-penneth and most of them will tell you everything you need to know for its every-day use. The MDN docs are a touch impenetrable, with some not intuitive use cases and weird terminology like 'initial value' and 'accumulator' crazying up your mind. Learning reduce (or becoming a Master of Reduction, to give this process its formal name) is one of those release moments in learning JavaScript.
Even now, there are arguments for going elsewhere - lodash's reduce, for example, can handle objects too.
Before then, people would have relied on their own or other implementations. The array method reduce is the best thing in JavaScript, but it's only existed as a native method of the language since 2009 or so, when the 5th Edition of ECMAScript was released.
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