Understanding Global & Local Scope in Javascript

Question

I\'ve been learning Javascript using Object-Oriented JavaScript by Stoyan Stefanov

He offers an example comparing global and local scope:

var a = 123         


        

Answer 1

It is not overriding the global variable. What is happening is called "variable hoisting". That is, a var a; gets inserted at the top of the function.

The script engine changes your script to be the following:

var a = 123;
function f() {
    var a;
    alert(a);
    a = 1;
    alert(a);
}
f();

Lesson to learn: Always declare your variables before you use them. Some will say declare all your variables at the top of the function.