What’s a derived state? Think one state for text and then another for uppercaseText.

Derived State

function Foo() {
    const [text, setText] = useState('hello, za warudo!');
    const [uppercaseText, setUppercaseText] = useState(text.toUpperCase());

    useEffect(() => {
        setUppercaseText(text.toUpperCase());
    }, [text])

    ...
}

Putting like that it’s crazy to think anyone would do this… right? RIGHT?

Yes, an example like this will make clear that this is wrong.

The Bad of Derived State

• Stored separately and out-of-sync with the actual state.
• Triggers (depend) on unnecessary re-renders.

How to refactor the derived state?

Say it’s an expensive calculation… the solution is to use useMemo.

function Foo() {
    const [text, setText] = useState('hello, za warudo!');
    const uppercaseText = useMemo(() => text.toUpperCase(), [text]);
    ...
}

How to quickly spot state that can be derived?

I came up with a good way of thinking that should make it easier to KNOW if a state should be “another state” or just a computed property (memorized or not depending on the case).

function Foo({
    text = 'hello, za warudo!',
    uppercaseText = text.toUpperCase(),
}) {
    ...
}

// Forget react for a moment...
// Would you ever call a function like this?
const text = 'hello, za warudo!';
Foo({
    text,
    uppercaseText: text.toUpperCase(),
});

If you think of those states as “props”, then this makes it more blatantly what it should be.

Forget React entirely, think only of functions:
Would you call a function with a variable and then another variable you could just compute inside?