How do you say that props are read only?

When you declare a component as a function or a class, it must never modify its own props.

Let us take a below capital function,

function capital(amount, interest) {
  return amount + interest;
}

The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."