By default when we create a variable without assigning anything, the variable will be undefined
Once we assigned it a value its harder to set it back to undefined as now it has a type and value for it.
We can do in following way using
var a; //a is undefined here a=5; // a is 5 a = void(0); //a is undefined again here
Here void states that the variable is of no type and 0 assigns to no value.