JS Global Variable to Local Variable

This is a simple question.

I know global variables are created when they are declared outside a function (says w3schools.com).

My question is, if I create a global variable and edit it in a function, does it become local? Does the new value given by the function become the global value?

Answers:

Answer

In general, no, editing a global does not make it local:

var myglob = 5;
function incGlob() {
    myglob = myglob + 1;
}

incGlob();
console.log(myglob); // is 6 now

However, if you pass the global variable as an argument, the argument is a local copy:

var myglob = 5;
function incArg(myloc) {
    myloc = myloc + 1;
}

incArg(myglob);
console.log(myglob); // is still 5

Note that objects are passed by reference, so editing the member variables of an argument variable changes the member variables of the original object passed in:

var myglob = { foo:5 };
function editLoc(myloc) {
    myloc.foo = 6;
}

editLoc(myglob);
console.log(myglob.foo); // foo is 6 now

Finally, note that the local variable in editLoc, above, is just a reference. If we try to overwrite the entire object (instead of a member variable), the function simply loses the reference to the original object:

var myglob = { foo:5 };
function clobberLoc(myloc) {
    myloc = { bar:7 };
}

clobberLoc(myglob);
console.log(myglob.foo); // myglob is unchanged...
// ...because clobberLoc didn't alter the object,
// it just overwrote its reference to the object stored in myglob 
Answer

No, editing the global variable will not change the variable's scope. The new value assigned becomes the global value.

http://jsfiddle.net/RtnaB/

myGlobal = 'foo'; // notice no 'var' keyword, implicitly global (DON'T DO THIS)

console.log(myGlobal); // logs 'foo'

var myFunc = function () {
    myGlobal = 'bar';
};

myFunc();

console.log(myGlobal); // logs 'bar'
Answer

Yes.

You will only create a local variable if you use the var keyword to declare it inside a function.

Answer

The new value becomes the global value.

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