In React.js, the "key" prop is a special attribute used to uniquely identify and distinguish individual elements within a list of components. When rendering dynamic lists, each component should have a unique key assigned to it. This key plays a crucial role in React's virtual DOM reconciliation process, enabling the library to efficiently update and render components based on changes in the underlying data. The key property is mostly used in Reactjs to render lists efficiently.

In ReactJs, the key prop doesn't just serve to identify elements within lists or mapped components uniquely. It also plays a crucial role in managing the state of components.

When a component mounts and unmounts within the same position in the DOM tree, React typically preserves its state. Consider this example:

isSelected ? <Input type="text" /> : <Input type="number" />;

Here, even though the Input component is being mounted and remounted based on the isSelected condition, its state won't be reset. To trigger a state reset during toggling, we can employ the key prop:

isSelected ? (
  <Input key="textInput" type="text" />
) : (
  <Input key="numberInput" type="number" />
);

By assigning distinct keys to each Input component, we instruct React to treat them as separate entities. This leads to a state reset when toggling occurs, ensuring that each input type maintains its independent state.