add exercise files

students/jan-gerrits/ex1.js

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+Array.isArray(input){
+    return Array.isArray(input)
+}
+
+
+/*var x = [1, 2, 4, 0];
+var n = x.includes("w3resource");
+var p = x.includes(0,1,2,4);
+
+console.log = n;
+console.log = p;
+
+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/jan-gerrits/ex2.js

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+var first =  function(array, n) {
+    if (array == null) 
+    return void 0;
+  if (n == null) 
+    return array[0];
+  if (n < 0)
+    return [];
+  return array.slice(0, n);
+};
+
+console.log(first([7, 9, 0, -2]));
+console.log(first([],3));
+console.log(first([7, 9, 0, -2],3));
+console.log(first([7, 9, 0, -2],6));
+console.log(first([7, 9, 0, -2],-3));
+
+
+
+/* 
+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/jan-gerrits/ex3.js

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+function joinArray(input) {
+    var index = 0,
+    result = '';
+
+    while(index < input.length)
+    result = result + ' ' + input[index];
+    index++;
+}
+
+return result;
+}
+/*
+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/jan-gerrits/ex4.js

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+function validatePIN(pin) {
+    if (typeof pin === "string" && !~pin.indexOf('.') && !isNaN(Number(pin)) && (pin.length === 4 || pin.length === 6)) { 
+      return true; 
+    } else { 
+      return false; 
+    }
+  }
+
+
+/* 
+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
+ */