Array average and range initialization

snippets/average-of-array-of-numbers.md

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+### Average of array of numbers
+
+Use `reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.
+
+```js
+var average = arr =>
+  arr.reduce( (acc , val) => acc + val, 0) / arr.length;
+```