Check undefined for Javascript

undefind validation | array + obj

/**
 * Returns true if key is not a key in object or object[key] has 
 * value undefined. If key is a dot-delimited string of key names,
 * object and its sub-objects are checked recursively.
 */
function isUndefinedKey(object, key) {
    var keyChain = Array.isArray(key) ? key : key.split('.'),
        objectHasKey = keyChain[0] in object,
        keyHasValue = typeof object[keyChain[0]] !== 'undefined';
    
    if (objectHasKey && keyHasValue) {
        if (keyChain.length > 1) {
            return isUndefinedKey(object[keyChain[0]], keyChain.slice(1));
        }
        
        return false;
    }
    else {
        return true;
    }
}

var undefined,
    ponies = {
        pretty: false,
        scummy: true,
        favorite: {
            food: 'money'
        },
        goodFor: undefined
    };

isUndefinedKey(ponies, 'pretty'); // false
isUndefinedKey(ponies, 'goodFor'); // true
isUndefinedKey(ponies, 'anyValueWhatsoever'); // true
isUndefinedKey(ponies, 'favorite.food'); // false

undefind validation | object

//To check for undefined-valued object keys

key in obj && typeof obj.key === 'undefined';
// - or -
obj.hasOwnProperty(key) && typeof obj.key === 'undefined';

undefind validation | variable

function isUndefined(value) {
    return typeof value === 'undefined';
}

var a, x = 4;

isUndefined(a); // true
isUndefined(x); // false