Cannot invoke an object which is possibly ‘undefined’.ts(2722)

With Typescript 3.7+, you can also use optional chaining to invoke the optional prop method:

const Button = (props: ButtonProps) => {
  const handleClick: React.MouseEventHandler<
    HTMLButtonElement | HTMLAnchorElement
  > = e => {
    props.onClick?.(e); // works
  };
};

You can read more about using optional chaining – https://www.stefanjudis.com/today-i-learned/optional-chaining-helps-to-avoid-undefined-is-not-a-function-exceptions/