[javascript] How to duplicate object properties in another object?

Given the object:

var firstObject = {
    key1 : 'value1',
    key2 : 'value2'
};

how can I copy the properties inside another object (secondObject) like this:

var secondObject = {
    key1 : 'value1',
    key2 : 'value2',
    key3 : 'value3',
    key4 : 'value4'
};

using a reference to the firstObject? Something like this:

var secondObject = {
    firstObject,
    key3 : 'value3',
    key4 : 'value4'
};

(this doesn't work... I put it just to show in big lines how I would like to structure the code).

Is a solution possible without using any JavaScript frameworks?