Create fill-array.md

snippets/fill-array.md

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+### Fill array
+
+Use `Array.map()` to map values between `start` (inclusive) and `end` (exclusive) to `value`.
+Omit `start` to start at the first element and/or `end` to finish at the last.
+
+```js
+const fillArray = (arr, value, start = 0, end = arr.length) => 
+  arr.map((v,i) => i>=start && i<end ? value : v);
+// fillArray([1,2,3,4],'8',1,3) -> [1,'8','8',4]
+```