Access outer function variables from inner function

Question

How to change value of x in function A() from function B()

function A() {
    var x = 10; // Value to be changed
    function B() {
        var x = 20;
        // From here i want to change the value of x (i.e. x=10 to x=40)
    }
    B();
}

A();

Answer 1

Do not use var when intending to overwrite the variable. Using var creates a new variable, local to the scope in which it is declared. That's why x is not changing on the outside.

function A() {
    var x = 10;
    function B() {
        x = 20; // change x from 10 to 20
    }

    B(); // x is now changed
}

Answer 2

If I understand your question, the following code is a solution:

function A() {
    var x = 10; // Value to be changed
    function B() {
        var x = 20;
        // From here i want to change the value of x (i.e. x=10 to x=40)
        changeX(40);
    }
    function changeX(y) {
        x = y;
    }
    B();
    alert(x);
}

A();

However there are more elegant approach, but it depends on your application.

Answer 3

Maybe:

function A() {
    var x = 10; // Value to be changed
    function B() {
        var x = 20;
        return x; // Return new value of x
    }
    x = B(); // Set x to result returned by B(), i.e. the new value
}

A();

Answer 4

the var statement will create new local variable. So in your example:

function A() {
    var x = 10; // You can think of it as A.x
    function B() {
        var x = 20; // And this as A.B.x
    }
}

Those two variables belong to different scope, if you want to access variable of an outer scope from within inner scope just access it, without re-declaring it.

You might want to checkout "JavaScript Language Specifications" document available at http://www.planetpdf.com/codecuts/pdfs/tutorial/jsspec.pdf to understand how scopes, statements and other basics work in JavaScript.