Function count the occurrences of substring in a string

occurrences.js

/** Function that count occurrences of a substring in a string;
 * @param {String} string               The string
 * @param {String} subString            The sub string to search for
 * @param {Boolean} [allowOverlapping]  Optional. (Default:false)
 *
 * @author Vitim.us https://gist.github.com/victornpb/7736865/edit
 * @see Unit Test https://jsfiddle.net/Victornpb/5axuh96u/
 * @see http://stackoverflow.com/questions/4009756/how-to-count-string-occurrence-in-string/7924240#7924240
 */
function occurrences(string, subString, allowOverlapping) {

    string += "";
    subString += "";
    if (subString.length <= 0) return (string.length + 1);

    var n = 0,
        pos = 0,
        step = allowOverlapping ? 1 : subString.length;

    while (true) {
        pos = string.indexOf(subString, pos);
        if (pos >= 0) {
            ++n;
            pos += step;
        } else break;
    }
    return n;
}

Thanks, I'm using your function in a php page.
I've included the url back to here.
Nice, clean, efficient code.
Well done!

how about "foofoofooooofo".split("foo").length

occurrences("This is very good", "", true) will give 18 which seems incorrect.

why this condition if (subString.length <= 0) return (string.length + 1);

occurrences("This is very good", "", true) will give 18 which seems incorrect.

why this condition if (subString.length <= 0) return (string.length + 1);

This is to prevent occurrences("asfsdfasdfasd", "") from looping infinitely.

It returns length+1 because:

> "abcd".match(new RegExp('', 'g'))
// ["", "", "", "", ""] (5)

Thanks for this! I would like to use your function in an open-source project. What is the license of your code?