Why does shadowed variable evaluate to undefined when defined in outside scope?

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Variables are subject to hoisting. This means that regardless of where a variable is placed within a function, it is moved to the top of the scope in which it is defined.

For example:

var outside_scope = "outside scope";
function f1() {
    alert(outside_scope) ;
    var outside_scope = "inside scope";
}
f1();

Gets interpreted into:

var outside_scope = "outside scope";
function f1() {
    var outside_scope; // is undefined
    alert(outside_scope) ;
    outside_scope = "inside scope";
}
f1();

Because of that, and the function only scope that JavaScript has, is recommended to declare all the variables at the top of the function, to resemble what will happen.

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