Generating a string which isn’t a property name of a given object

Helping out with the test262 test suite (official ECMAScript 5 test suite), I have come to a problem. Based on a given object, find/generate a string which isn’t a property name of the object.

Here is the solution I’ve come up with

function unusedPropertyName(o){  
    var ownPropNames = Object.getOwnPropertyNames(o);

    var unusedName = ownPropNames.reduce(function(prev, curr, i){  
        var A = ‘a’, B = ‘b’;  
        var l = curr[i];  
        l = (l=== undefined || l !== A) ? A : B;

        return prev + l;  
    }, ”);

    return unusedName;  
}

The idea is to list all property names. Then, create a string which has one letter that differs from each property name. This idea is inpired by Cantor’s diagonal argument.

I found it was an interesting use of Array.prototype.reduce and was worth sharing.