add array flat method example

JavaScript_Advance/arrayFlat.js

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+const array = [];
+array.flat(); // This is array flat method came new in ES-2019
+
+
+/**
+ * The flat() method creates a new array with all sub-array elements concatenated into 
+ * it recursively up to the specified depth.
+ * 
+ */
+
+const arr = [[1, 2, 3], ['a', 'b', 'c']]
+
+console.log(arr.flat()) // [1,2,3,'a','b','c']
+
+const arr1 = [[1, 2, 3], ['a', ['b', 'c']]]
+
+// My default flat only work one level deep
+console.log(arr1.flat()) // [1,2,3,'a', ['b','c']]
+
+// if you want to work on more level you can pass level etc. 2, 3
+console.log(arr1.flat(2)) // [1,2,3,'a', 'b','c']
+
+// if you want to work on n level you need to pass Infinity
+console.log(arr1.flat(Infinity)) // [1,2,3,'a', 'b','c']

docs/JavaScript_Advance/arrayFlat.md

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+# Array Flat Method