Javascript: How to implement iterator over multidimensional array?

I have 2 dim array, which looks like this:

var a = [[1,2,3],[4,5,6],[7,8,9]];

I want to write an iterator which will return one value a time when it's called.

iterator(); //returns 1
iterator(); //returns 2
iterator(); //returns 3

I tried such approach:

function iterator() {
    var a = [[1,2,3],[4,5,6],[7,8,9]];
    var i, j;
    return function() {
        for (var i = 0; i < a.length; i++) {
            var b = a[i];
            for (var j = 0; j < b.length; j++) {
                return a[i][j];
            }
        }
    }
};
var a = iterator();
a(); //1
a(); //1
a(); //1

It always returns me first element.

I can try this one:

function iterator() {
    var a = [[1,2,3],[4,5,6],[7,8,9]];
    var i = 0, j = 0;
    return function() {
        for (; i < a.length; i++) {
            var b = a[i];
            for (; j < b.length; j++) {
                return a[i][j];
            }
        }
    }
};

Also not works.

But if I try this one:

function test() {
    var a = [1,2,3,4,5], i = 0;
    return function() {
        while (i < a.length) {
            return a[i++];
        }
    }
}
var a = test();
a(); //1
a(); //2
a(); //3

It works fine.

What is the difference here? How to make for loop work?

One other visible problem for me is bounds. How should I stop when I reach array bounds?

Instead of using an inner for-loop for the second dimension you can use a simple if to test the bounds of j

function iterator() {
    var a = [[1,2,3],[4,5,6],[7,8,9]];
    var i = 0, j = 0;
    return function() {
        for (; i < a.length; i++) {
            var b = a[i];
            if (j < b.length){
                return a[i][j++];
            }
            j = 0;
        }
        return undefined; // reached when there is no value left
    }
};