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