Added Array Flat Method to close #198

docs/JavaScript_Advance/array_flat.md

@@ -1 +1,24 @@
-# Array Flat Method
+# Array Flat Method
+
+The `flat()` method creates a new array with nested sub-array elements concatenated into it recursively up to the specified depth. The default depth is one level but the `Infinity` parameter may be used to recursively flatten an array completely until no nested arrays remain.
+
+```js
+var array1 = [1, 2, [3, 4]];
+array1.flat();
+// [1, 2, 3, 4]
+```
+```js
+var array2 = [1, 2, [3, 4, [5, 6, [7, 8]]]];
+array2.flat(2);
+// [1, 2, 3, 4, 5, 6, [7, 8]]
+```
+```js
+var array3 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
+array3.flat(Infinity);
+// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+```
+```js
+var array4 = [1, 2, [3, 4], [5, 6], [7, 8, [9, 10]]];
+array4.flat();
+// [1, 2, 3, 4, 5, 6, 7, 8, [9, 10]]
+```