Can I use variables from outside the function?

Answer 1

Yes that's because in your code 'Lastname' is a global variable.

Answer 2

A good question,
The scope of the variable matters. The variable can only be accessed within its scope. There are two types of variable scope in JavaScript: Global Scope and Local Scope.

var a=10; //Global variable a

function sum(number){

// Scope of variable b starts

var b=8;

// Loval variable b
return number+b;
}

//scope of variable b ends

alert(a); //10
alert(b); //undefined
alert(sum(a)); //18

Answer 3

YES, I tried it with the following code:

var Lastname="Beckham";
function sayHello(name) {
alert("Hi, " + name + " " + Lastname);
}

sayHello("David");

// In the popup window shows: Hi, David Beckham