Add exercise files

students/rename-this-folder/ex1.js

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+/*
+Write a JavaScript function to check whether an input is an array or not.
+
+Test Data :
+console.log(isArray('w3resource')); // false
+console.log(isArray([1, 2, 4, 0])); // true
+*/
+
+

students/rename-this-folder/ex2.js

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+/* 
+Write a JavaScript function to get the first element of an array. Passing a parameter 'n' will return the first 'n' elements of the array.
+
+Test Data : 
+console.log(first([7, 9, 0, -2])); // 7
+console.log(first([],3)); // []
+console.log(first([7, 9, 0, -2],3)); // [7,9,0]
+console.log(first([7, 9, 0, -2],6)); // [7, 9, 0, -2]
+console.log(first([7, 9, 0, -2],-3)); // [] 
+
+*/

students/rename-this-folder/ex3.js

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+/*
+Write a simple JavaScript program to join all elements of the following array into a string.
+
+Sample array : 
+var myText = ['This', 'is', 'not', 'working'];
+joinArray(myText);
+
+Expected Output : 
+This is not working
+ */

students/rename-this-folder/ex4.js

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+/* 
+ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 4 digits or exactly 6 digits. If the function is passed a valid PIN string, return true, else return false. 
+
+validatePIN(1234); // false
+validatePIN(00004324); // false
+validatePIN('2312'); // true
+validatePin('asd123'); // false
+validatePin('000010'); // true
+ */