Symtoms

Uncaught TypeError: *** is not a function in a React component
or
someState.map is undefined

Cause

someState Array data is not loaded before rendering.

Solution

render components with states after data is all loaded.

const [ loadState, setLoadState ] = React.useState(true);
React.useEffect( () => {
    setLoadState(true);
    setDataState(data);
    setLoadState(false);
},[]);
if (loadState == true){
    return( <>loading...</>);
}
return( 
    ...render components...
);