here is the offending code from the downvotePost method

actions.js

export function downvotePost(id) {
  var myHeaders = { headers: { 'Authorization': 'cakelolgarbage', 'Content-Type': 'application/json' }}
  var requestBody = { 'option': 'downVote'}

  var myInit = { method: 'POST',
               headers: myHeaders,
               body: requestBody,
               mode: 'cors',
               cache: 'default' }

  console.log(myInit)

  return (dispatch, id) => {
    console.log(dispatch)
    console.log(id)
    dispatch(downvote(id))
    return fetch(`http://localhost:3001/posts/{id}`, myInit )
      .then(response => {
        console.log(response)
        return response.json()
      })
      .then(json => dispatch(receivePosts(json)))
  }
}

I think line 13 should just be
return dispatch =>

if you look at export function fetchPostsIfNeeded(subreddit) in the tutorial that you pointed me to, you'll see the parameter subreddit is used within the return anon function without having to pass the parameter in. I think you similarly should be able to use id from line #1 in block 13-23 without having to pass it as a parameter