Create Flatten.md

Author: ps0305

FE75/Flatten.md

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+```js
+/**
+ * Flattens a nested array into a single-level array.
+ *
+ * @param {Array} array - The array to flatten.
+ * @returns {Array} - A new array with all sub-array elements flattened.
+ */
+function flatten(array) {
+  // Validate input to ensure it's an array
+  if (!Array.isArray(array)) {
+    throw new TypeError('Expected an array');
+  }
+
+  const result = []; // Create a new array to store the flattened elements
+
+  // Helper function to recursively flatten the array
+  function recursiveFlatten(arr) {
+    for (let i = 0; i < arr.length; i++) {
+      const item = arr[i];
+      if (Array.isArray(item)) {
+        // If the item is an array, recurse into it
+        recursiveFlatten(item);
+      } else {
+        // If it's not an array, push it to the result
+        result.push(item);
+      }
+    }
+  }
+
+  recursiveFlatten(array); // Start the recursive flattening
+  return result; // Return the flattened array
+}
+
+export default flatten;
+```
+### Explanation
+#### Validate Input:
+The function verifies that the input array is indeed an array using Array.isArray and throws an error if it's not.
+
+#### Create a result Array:
+A new array (result) is created to store the flattened elements. This ensures that the original input array remains unchanged (pure function).
+
+#### Recursive Helper Function:
+A recursive helper function, recursiveFlatten, is defined to iterate through the array:
+
+* If an element is an array (Array.isArray(item)), the function recursively calls itself with the sub-array.
+* If the element is not an array, it is directly pushed into the result array.
+#### Return the Result:
+After the recursive flattening is complete, the result array is returned.