What is the root cause of the error: TypeError: Cannot read property ‘key’ of undefined

One can find this error type in React, Angular, React-Native. I am able to find a lots of questions with this error type but I am curious as to when does the console throw this error?

Edit:
Can you explain me with the following reference to react-native

export class EventList extends React.Component {
render() {
    return (
        <FlatList 
            data={[{key: 'a'},{key: 'b'}]}
            renderItem={({ itemObject }) => <Text>{itemObject.key}</Text>}
        />
    );
}
}

Thanks in advance:)

Source: AngularJS