Surprised that global variable has undefined value in JavaScript

This phenomenon is known as: JavaScript Variable Hoisting.

At no point are you accessing the global variable in your function; you're only ever accessing the local value variable.

Your code is equivalent to the following:

var value = 10;

function test() {
    var value;
    console.log(value);

    value = 20;
    console.log(value);
}

test();

Still surprised you're getting undefined?

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Explanation:

This is something that every JavaScript programmer bumps into sooner or later. Simply put, whatever variables you declare are always hoisted to the top of your local closure. So, even though you declared your variable after the first console.log call, it's still considered as if you had declared it before that.
However, only the declaration part is being hoisted; the assignment, on the other hand, is not.

So, when you first called console.log(value), you were referencing your locally declared variable, which has got nothing assigned to it yet; hence undefined.

Here's another example:

var test = 'start';

function end() {
    test = 'end';
    var test = 'local';
}

end();
alert(test);

What do you think this will alert? No, don't just read on, think about it. What's the value of test?

If you said anything other than start, you were wrong. The above code is equivalent to this:

var test = 'start';

function end() {
    var test;
    test = 'end';
    test = 'local';
}

end();
alert(test);

so that the global variable is never affected.

As you can see, no matter where you put your variable declaration, it is always hoisted to the top of your local closure.

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Side note:

This also applies to functions.

Consider this piece of code:

test("Won't work!");

test = function(text) { alert(text); }

which will give you a reference error:

Uncaught ReferenceError: test is not defined

This throws off a lot of developers, since this piece of code works fine:

test("Works!");

function test(text) { alert(text); }

The reason for this, as stated, is because the assignment part is not hoisted. So in the first example, when test("Won't work!") was run, the test variable has already been declared, but has yet to have the function assigned to it.

In the second example, we're not using variable assignment. Rather, we're using proper function declaration syntax, which does get the function completely hoisted.

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Ben Cherry has written an excellent article on this, appropriately titled JavaScript Scoping and Hoisting.
Read it. It'll give you the whole picture in full detail.