if we declare but not use then its undefined if we didnot declare then its null
1. console.log(a); /* undefined */var b;2. console.log(b); /* null */
In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, null is an assignment value. It can be assigned to a variable as a representation of no value:
undefined is a type itself (undefined) while null is an object.