✨ Add "inventory-update" challenge

challenges/inventory-update/solutions/javascript-inventory/README.md

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+# javascript-inventory - inventory-update
+
+Programming language : JavaScript
+Created by [@Divlo](https://github.com/Divlo) at 8 July 2020.

challenges/inventory-update/solutions/javascript-inventory/solution.js

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+function solution (array1, array2) {
+  const result = [...array1]
+
+  array2.forEach(delivery2 => {
+    const foundResultIndex = result.findIndex(
+      deliveryResult => deliveryResult.item === delivery2.item
+    )
+    if (foundResultIndex === -1) return result.push(delivery2)
+    return (result[foundResultIndex].quantity =
+      result[foundResultIndex].quantity + delivery2.quantity)
+  })
+
+  return result.sort((a, b) => a.item.localeCompare(b.item))
+}
+
+module.exports = solution