useFileHandlers.js

const reducer = (state, action) => {
  switch (action.type) {
    case 'load':
      return { ...state, files: action.files, status: LOADED }
    case 'submit':
      return { ...state, uploading: true, pending: state.files, status: INIT }
    case 'next':
      return {
        ...state,
        next: action.next,
        status: PENDING,
      }
    case 'file-uploaded':
      return {
        ...state,
        next: null,
        pending: action.pending,
        uploaded: {
          ...state.uploaded,
          [action.prev.id]: action.prev.file,
        },
      }
    case 'files-uploaded':
      return { ...state, uploading: false, status: FILES_UPLOADED }
    case 'set-upload-error':
      return { ...state, uploadError: action.error, status: UPLOAD_ERROR }
    default:
      return state
  }
}