difference between null and undefined?

Question

please describe with an example.
thanks in advanced

Answer 1

Javascript is a mess man. Both mean 'nothing' so in practice there's not much of a difference, and it's ok to be confused about the two.

Technically though, variables are `undefined` if they have been declared but haven't been assigned a value yet. undefined also gets it's own type, "undefined".

var a;
console.log(a); // undefined
console.log(typeof(a)); // "undefined"

If variables haven't even been declared, using them throws.

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

`null` is an actual thing you assign to variables.
var a;
a = null;
console.log(a); // null
console.log(typeof(a)); // "object"

`null` is not a real object though, as you know when you try to access a property of it:
var a = null;
console.log(a.lol); // Uncaught TypeError: cannot read property 'lol' of null

You can also assign `undefined` to a variable so that muddies the water a bit, and I don't really recommend it.
var a = 4;
a = undefined;
console.log(typeof(a)); // "undefined"

Bottom line: use undefined for undefined checks (eg `if(a === undefined)`) and null for everything else, especially never assign undefined to a variable.

Also note that `null == undefined`, but `null !== undefined`.