reactjs - What does the colon mean in Javascript after function?

Question

I saw the Facebook F8 app code there is a ":" after the function

function setup(): React.Component {
  ...
}

What does this mean?

Inheritance?

Answer 1

Usually it's flowtype annotation and in this case means that setup() returns a React.Component. Or it could be TypeScript as well, can't make sophisticated guess in this particular case.