underscore valid object chain and truth test

gistfile1.txt

_.mixin({
    validChain: function (chain, test) {
        var parts = chain.split('.'), valid = false, struct = window;
        
        if (parts[0] === 'window') parts.splice(0, 1); 
        valid = _.all(parts, function (part) { return struct = struct[part]; });
 
        if (test && valid) {
            if (typeof test === 'function')
                return test(struct);
            else    
                return struct === test;
        } else {
            return valid;                            
        }   
    }
});