JavaScript's function scope and declaration in advance

Don't say a word; first a piece of code;

var scope= ' global '; function test () {alert (scope);      Output undefine, not global var scope= ' local ';        alert (scope); Output local}test ();

You may think that the first alert will output global;

But in fact the first alert output is undefined;

What's going on here?

To explore this problem first of all to explain the next two concepts;

1: Function Scope:

function scope refers to the variables declared within the function are only valid within this function;

2: Declaration in advance:

Declaration in advance means that a variable declared anywhere within a function will be first parsed at the top of the function;

After understanding the concept of declaration in advance, the above code is equivalent to the following;

If this statement is not literally understood in advance, then it is easy to understand from the code;

var scope= ' global '; function test () {var scope;        The variable alert (scope) is declared first.       Because it is not yet assigned, so it is undefine scope= ' local ';        Assignment alert (scope); Output local}test ();

So the experience is that in the function body first the variables to be used are declared at the beginning of the function;

Instead of declaring variables when they are used;

As to why the first alert is not an output global;

Because the local variables in the function body will obscure the global variables;

Must not explain too much;

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Function scope and declaration of JavaScript in advance