javascript undefined typeof null boolean

Tut#3 JavaScript Data Types Part 1: Undefined, Null, Boolean and TypeOf

When does a variable get the value of undefined? How and when to use the special value ‘null’ in JavaScript? Why null==undefined in JavaScript? How to use a variable with only 2 values: true and false termed as a boolean in JavaScript? How to use the typeof operator to determine the data type of a given variable? What are the different rules of conversion of a variable to a boolean in JS? What values does the typeof operator return for each data type in JS?