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// FROM: http://jsfiddle.net/DWefK/5/

var obj = {
    a: {
        b: ''
    }
};

function defined(ref, strNames) {
    var name;
    var arrNames = strNames.split('.');
    
    while (name = arrNames.shift()) {        
        if (!ref.hasOwnProperty(name)) return false;
        ref = ref[name];
    } 
    
    return true;
}

console.log( defined( obj, 'a.b' ) );