diff --git a/01_Day_Introduction/01_day_starter/datatypes.js b/01_Day_Introduction/01_day_starter/datatypes.js
new file mode 100644
index 0000000..4effec6
--- /dev/null
+++ b/01_Day_Introduction/01_day_starter/datatypes.js
@@ -0,0 +1,60 @@
+// typeof operator, checks datatype of variables
+
+console.log(typeof character);
+console.log(typeof characterIQ);
+console.log(typeof characterIsMarried);
+console.log(typeof testOfUndefined);
+console.log(typeof emptyValue);
+
+// variables without values
+
+let test1;
+let test2;
+let test3;
+let test4;
+
+console.log(test1, test2, test3, test4);
+
+// variables with assigned values
+
+const varWithValue = "assigned value";
+let test = false;
+console.log(test);
+test = true;
+const num = 100;
+let operator;
+console.log(varWithValue, test, num, operator);
+
+// variables in multiple lines
+
+let characterFirstName = "Penny";
+let characterLastName = "Hofstadter";
+let characterIsMarriedStatus = true;
+let characterCountry = "USA";
+let characterAgeOf = 22;
+
+console.log(
+  characterFirstName,
+  characterLastName,
+  characterIsMarriedStatus,
+  characterCountry,
+  characterAgeOf
+);
+
+//variables in single line
+
+let firstNameTest = "Max",
+  lastNameTest = "Mustermann",
+  isMarriedTest = false,
+  countryTest = "Musterland",
+  ageTest = null;
+
+console.log(firstNameTest, lastNameTest, isMarriedTest, countryTest, ageTest);
+
+// age variables with initial values
+
+let myAge = 35;
+let yourAge = 74;
+
+console.log("I am " + myAge + " years old.");
+console.log("You are " + yourAge + " years old.");
diff --git a/01_Day_Introduction/01_day_starter/helloworld.js b/01_Day_Introduction/01_day_starter/helloworld.js
index 8c9e2c0..24601a1 100644
--- a/01_Day_Introduction/01_day_starter/helloworld.js
+++ b/01_Day_Introduction/01_day_starter/helloworld.js
@@ -1 +1,13 @@
-console.log('Hello, World!')
\ No newline at end of file
+console.log("Hello, World!");
+
+// comments
+
+// single comment: comments can make code readable
+
+// Welcome to 30DaysOfJavaScript
+
+/*
+multiline comment:
+comments can make code readable, 
+easy to reuse and informative
+*/
diff --git a/01_Day_Introduction/01_day_starter/index.html b/01_Day_Introduction/01_day_starter/index.html
index 7ed5b19..ef45505 100644
--- a/01_Day_Introduction/01_day_starter/index.html
+++ b/01_Day_Introduction/01_day_starter/index.html
@@ -1,19 +1,17 @@
 <!DOCTYPE html>
 <html lang="en">
+  <head>
+    <title>30DaysOfJavaScript</title>
+  </head>
 
-<head>
-	<title>30DaysOfJavaScript</title>
-</head>
-
-<body>
-	<h1>30DaysOfJavaScript:03 Day</h1>
-	<h2>Introduction</h2>
-	<button onclick="alert('Welcome to 30DaysOfJavaScript!');">Click Me</button>
-	<script src="./helloworld.js"></script>
-	<script src="./introduction.js"></script>
-	<script src="./variable.js"></script>
-	<script src="./main.js"></script>
-
-</body>
-
-</html>
\ No newline at end of file
+  <body>
+    <h1>30DaysOfJavaScript:03 Day</h1>
+    <h2>Introduction</h2>
+    <button onclick="alert('Welcome to 30DaysOfJavaScript!');">Click Me</button>
+    <script src="./helloworld.js"></script>
+    <script src="./introduction.js"></script>
+    <script src="./variable.js"></script>
+    <script src="./main.js"></script>
+    <script src="./datatypes.js"></script>
+  </body>
+</html>
diff --git a/01_Day_Introduction/01_day_starter/variable.js b/01_Day_Introduction/01_day_starter/variable.js
index 62558cb..064b931 100644
--- a/01_Day_Introduction/01_day_starter/variable.js
+++ b/01_Day_Introduction/01_day_starter/variable.js
@@ -1,20 +1,37 @@
 // Declaring different variables of different data types
 
-let firstName = 'Asabeneh'    // first name of a person
-let lastName = 'Yetayeh'      // last name of a person
-let country = 'Finland'       // country
-let city = 'Helsinki'         // capital city
-let age = 100                 // age in years
-let isMarried = true
+let firstName = "Asabeneh"; // first name of a person
+let lastName = "Yetayeh"; // last name of a person
+let country = "Finland"; // country
+let city = "Helsinki"; // capital city
+let age = 100; // age in years
+let isMarried = true;
 
 // Declaring variables with number values
 
-const gravity = 9.81         // earth gravity  in m/s2
-const boilingPoint = 100     // water boiling point, temperature in oC
-const PI = 3.14              // geometrical constant
+const gravity = 9.81; // earth gravity  in m/s2
+const boilingPoint = 100; // water boiling point, temperature in oC
+const PI = 3.14; // geometrical constant
 
 // Variables can also be declaring in one line separated by comma
 
-let name = 'Asabeneh',     //name of a person
-  job = 'teacher',
-  live = 'Finland'
+let name = "Asabeneh", //name of a person
+  job = "teacher",
+  live = "Finland";
+
+// my own variables
+
+let character = "Sheldon Cooper";
+console.log(character);
+
+character = "Leonard Hofstadter";
+console.log(character);
+
+const characterIQ = 173;
+console.log(characterIQ);
+console.log(character, characterIQ);
+
+const characterIsMarried = true;
+let testOfUndefined;
+let emptyValue = null;
+console.log(characterIsMarried, testOfUndefined, emptyValue);
