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;

Answer 2

undefined: Declared but the value not asigned above where we access it in othe word it exist but the value not asigned

'not defined': It is an error which indicate to the coder/programmer/user the variable doesn't exist in the scope. Not declare in scope.

var a = 1;
console.log(a) //1

var b;
console.log(b) //undefined

console.log(c) //undefined
var c=2;

console.log(d) //undefined
var d;

e=3
console.log(e) //3
var e;

console.log(f) //Uncaught ReferenceError: f is not defined

> Note: the default behavior of javaScript moving declarations to the
> top
>So the above code is look like below internally

var a = 1, b, c, d, e;
console.log(a) //1

console.log(b) //undefined

console.log(c) //undefined
c=2;

console.log(d) //undefined

e=3
console.log(e) //3

console.log(f) //Uncaught ReferenceError: f is not defined