What is the difference in Javascript between 'undefined' and 'not defined'?

Question

If you attempt to use a variable that does not exist and has not been declared, javascript will throw an error. var name is not defined, and the script will sto

Answer 1

alert(a);

Here a is not defined.

alert(a);
var a=10;

Here a is undefined because javascript engine convert this code to

var a;
alert(a); // that's why `a` is defined here 
a=10;