redux-thunk middleware multiple => Syntex

Question

I was trying to understand how thunk works under the shell, but struggling to understand this code referenced from here

function createThunkMiddleware(extraArgument) {
  return ({ dispatch, getState }) => next => action => {
    // This gets called for every action you dispatch.
    // If it's a function, call it.
    if (typeof action === 'function') {
      return action(dispatch, getState, extraArgument);
    }

    // Otherwise, just continue processing this action as usual
    return next(action);
  };
}

const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;

export default thunk;

It says this is basically the code for thunk. What confuses me here is the multiple => syntax. I know => is part of the arrow functions. My current understanding is that createThunkMiddleware returns a function, let's call it A, that returns another function B, that returns another function C. And the signatures of A, B and C are like below:

function A ({dispatch, getState}) {
  return B(next)
}
function B (next) {
  return C(action)
}
function C (action) {
  ....
}

But this doesn't make sense to me. Because, how does A pass next to B, same for B.

Answer 1

redux-thunk is just a normal middleware of redux. There exists some type of control flow in middleware system. please check here , and you'll know how this come out.