Is React smart enough to bypass setState if I pass in the same value as the current state?

Question

I am setting multiple states in setState, but one of the states will not always change though. Is React smart enough to bypass setting that state it remains unchanged?

Answer 1

No, by default React will update your component with every setState call.
You can use shouldComponentUpdate to let the component know that it shouldn't update when certain conditions are met.

shouldComponentUpdate(nextProps, nextState) {
    // If these match, we know we don't need a redraw
    if (this.state.foo === nextState.foo) return false;
    // Otherwise, continue with the update
    return true;
}

Note that shouldComponentUpdate doesn't affect child-components, who will each check their own update conditions when they receive new props or new state.

You can read more about the React component lifecycle over here.