[sebmarkbage]

The difference is that if you use Object.observe to track changes, then you don't have the ability to hold onto the previous version of an object and know what the old value was.

This is important for animations where you want to animate from the object's old value to the new value.

[al2o3cr]

The objects passed to the Object.observe callback function describe both the new value and the old value, FWIW.