Empty shouldComponentUpdate works, otherwise not?

Question

I really can't figured out why if I declare an empty shouldComponentUpdate in a component, like this:

shouldComponentUpdate: function(nextProps, nextState) {},

React automatically doesn't render the component when it's not necessary (and that's perfectly fine).

WHILE if I remove the empty declaration, it will render it every time...

I'm using Immutable.js for props.

Answer 1

From the docs

By default, shouldComponentUpdate always returns true to prevent subtle bugs when state is mutated in place [...]

If you override it with an empty function, it will return undefined, which is cast to false, therefore your component never re-renders (except in the case where a forceUpdate will force it to render without checking shouldComponentUpdate).

Answer 2

Oscar is correct, you are getting an undefined. Try

shouldComponentUpdate: function(nextProps, nextState) { return true; },