react issue – Cannot assign to read only property '0' of object '[object Array]' —

What causes the ff issue ? Cannot assign to read only property ‘0’ of object ‘[object Array]’ ?

Any idea would be appreacited. Thanks.

#ts code snippet

 const [RegionalList, setRegionalList] = useState<IRegionalList[]>(RegionalListData);


 const setEmailValue = (event: any, regionalId: number, index: number) => {
    setRegionalList((prevState: IRegionalList[]) => {
      const newState = prevState.map((prop: IRegionalList) => {
        if (prop.id === regionalId) {
          prop.emails[index] = { emailAddress: event.target.value, id: null };
          return { ...prop };
        }
        return prop;
      });
      return newState;
    });
  }

>Solution :

here is the way that I suggest :

 const [RegionalList, setRegionalList] = useState<IRegionalList[]>(RegionalListData);


 const setEmailValue = (event: any, regionalId: number, index: number) => {
    setRegionalList((prevState: IRegionalList[]) => {
      const newState = prevState.map((prop: IRegionalList) => {
        if (prop.id === regionalId) {
         return { ...prop,emails:[...prop.emails.filter( (_,i)=>i !== index ),{ emailAddress: event.target.value, id: null }] } 
        }
        return prop;
      });
      return newState;
    });
  }

and if you care about order of your list you can do this :

    const setEmailValue = (event: any, regionalId: number, index: number) => {
    setRegionalList((prevState: IRegionalList[]) => {
      const newState = prevState.map((prop: IRegionalList) => {
        if (prop.id === regionalId) {
         let emails = prop.emails;
         emails[index] = { emailAddress: event.target.value, id: null };
          return { ...prop,emails }
        }
        return prop;
      });
      return newState;
    });
  }

please let me know if it fixed your problem