Added allUnique snippet

snippets/allUnique.md

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+---
+title: allUnique
+tags: array,beginner
+---
+
+Checks if all elements in an array are equal and returns`true` else returns `false`.
+
+- Use `Array.prototype.length` property to return the number of elements in that array.
+- Use `Set()` object to collect unique elements from the given array and then the `Set.prototype.size` to get the number of elements in the set
+- Check the two values for equality
+
+```js
+const allUnique = myArray => myArray.length === new Set(myArray).size;
+```
+
+```js
+allUnique([1,2,3,4,5]); // true
+allUnique([1,1,2,3,4]); // false
+```