React Function component setting state variable V/s using local variable

user_485743 0
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I have a functional component and want to update some state in that component (based on some context API object value i.e. I get via useContext)

Now, I see 2 options;

  1. Using local variable
  2. Using setState

I just wanted to understand the difference between the 2 ways. Also to add, when using setState, I run into below issue Error: Too many re-renders. React limits the number of renders to prevent an infinite loop.

Just to add, this functional component is a direct child of App component and the context object is updated in another component which is also a direct child of App component.

const MyComponent = () => {
    let myTitle = "ABC"; //Option 1 to use 'myTitle'
    //const [myTitle, setMyTitle] = useState('ABC'); //Option 2 to use 'myTitle'
    if (someContextApiObj.value === "XYZ") {
        myTitle = "XYZ";
        //setMyTitle("XYZ")
    }

    return (
        <>
            <ChildComponent
                myTitle={myTitle}
            />

        </>
    );
};

export default MyComponent;
react-hooks reactjs
3 Answers

Your code runs into an infinite loop because of your if-clause. If you set the title and the condition doesn't change, it will get set on every rerender causing a new render - infinite chain of rerenders.

Using a state offers you the ability to change the value conditionally, while using a local variable not coupled to a state makes the variable constant.

user_82169290

You should use the Hook useEffect.

const MyComponent = () => {
    const [myTitle, setMyTitle] = useState('ABC'); //Option 2 to use 'myTitle'

    useEffect(()=> {
        if (someContextApiObj.value === "XYZ") {
            setMyTitle("XYZ")
        }
    }, []);
    

    return (
        <>
            <ChildComponent
                myTitle={myTitle}
            />

        </>
    );
};

export default MyComponent;
user_25326950

It seems you are wanting to compute a myTitle value based on some prop or context value being passed to the component. Storing passed props/context values in local state is generally considered anti-pattern in React. Compute the myTitle value and pass it along to the child component.

const MyComponent = () => {
  const myTitle = someContextApiObj.value === "XYZ" ? "XYZ" : "ABC";

  return <ChildComponent myTitle={myTitle} />;
};

If the myTitle value is an expensive calculation or you just want to provide a stable reference to children, use the useMemo hook to provide this.

useMemo

import { useMemo } from 'react';

const MyComponent = () => {
  const myTitle = useMemo(() => {
    return someContextApiObj.value === "XYZ" ? "XYZ" : "ABC";
  }, [someContextApiObj]);

  return <ChildComponent myTitle={myTitle} />;
};
user_86908570
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