How can I make the variable newUser defined inside

Question

I send the variable newUser from options.html to background.html with chrome.extension.getBackgroundPage() like this

document.getElementById("save").addEventListener(
    "click", function ()
    {
        var newUser = document.getElementById("getEmail").value;
        var bkg = chrome.extension.getBackgroundPage();
        bkg.saveNewUser(newUser);
        console.log("newUser " + newUser);
    } , false)

In background.html I have

function saveNewUser (newUser)
{
    newUser = newUser; //this should be global?
    console.log("newUser from options page " + newUser);
}

console.log("newUser from options page " + newUser);

but newUser is local to the function. My understanding is that if a variable is saved inside a function without the keyword var it should be global. But in this case it is not. The console.log outside the function throws Uncaught ReferenceError: newUser is not defined error.

What am I doing wrong and how can I fix the scope to use newUser outside the function?

Thanks.

Edit

I tried the following in background.html but I still get newUser undefined error:

var extension_user

function saveNewUser (newUser)
{
    extension_user = newUser; //this should be global
    console.log("extension_user from options page " + extension_user);
}

console.log("extension_user from options page " + extension_user);

Update

I changed both pages to reflect the answers and comments but still outside the function the variable is undefined. What am I doing wrong?

options.html

document.getElementById("save").addEventListener(
    "click", function ()
    {
        var newUserEmail = document.getElementById("getEmail").value;
        var bkg = chrome.extension.getBackgroundPage();
        bkg.saveNewUser(newUserEmail);
        console.log("newUserEmail " + newUserEmail);
    } , false)

background.html

var newUser

function saveNewUser (newUserEmail)
{
    window.newUser = newUserEmail; //this should be global
    console.log("newUser from options page inside function" + newUser);
}

console.log("newUser from options page " + newUser);

Answer 1

The newUser variable does not exist in the global scope until the function saveNewUser is called. Thus it gives a reference error. Just declare the variable newUser in the global scope as var newUser;. That should solve the problem. Always use the var keyword when declaring variables. It's a good programming practice. Never declare global variables within local scopes. You already know why. Please vote for me if this answer helped you.

Edits:

Here's the code to clarify my answer:

var newUser; // new user declared as a global

function saveNewUser (newuser)
{
    newUser = newuser; // this should be global?
    console.log("newUser from options page " + newUser);
}

console.log("newUser from options page " + newUser); // no more reference error

Answer 2

If you want to save it as a global variable, you can either change the name of the variable in the function:

function saveNewUser (newUserVar)
{
    newUser = newUserVar; //this should be global?
    console.log("newUser from options page " + newUserVar);
}

console.log("newUser from options page " + newUser);

or you can use the window scope (which is the same as the global scope):

function saveNewUser (newUser)
{
    window.newUser = newUser; //this should be global?
    console.log("newUser from options page " + newUser);
}

console.log("newUser from options page " + newUser);

All global variables are essentially properties of the window object.