To get the reaction you have to use the field resolver reaction passing in a profileId.

Below is a publications query that gets all the publications for profile 0x09. In the reaction field resolver you can pass in a profileId you wish to know the reaction state for, most apps would use the logged in users selected profile they are browsing on.

API design

please note the example below doesn't pick all the content out of the publication it just shows you the field used to get that back.

query Publications {
  publications(request: {
    profileId: "0x09",
    publicationTypes: [POST, COMMENT, MIRROR],
    limit: 10,
  }) {
    items {
      __typename 
      ... on Post {
        reaction(request: { profileId: "0x01" })
      }
      ... on Comment {
        reaction(request: { profileId: "0x01" })
      }
      ... on Mirror {
        reaction(request: { profileId: "0x01" })
      }
    }
    pageInfo {
      prev
      next
      totalCount
    }
  }
}

profileId for reaction can pass in as a variable easily enough as well. You can imagine passing the logged-in users profile they are browsing on to see if they have reacted to the publication. This can be hooked in like this for every query which returns a publication type (Post or Comment or Mirror)

query Publications($publicationsRequest: PublicationsQueryRequest!, $reactionRequest: ReactionFieldResolverRequest) {
  publications(request: $publicationsRequest) {
    items {
      __typename 
      ... on Post {
        reaction(request: $reactionRequest)
      }
      ... on Comment {
        reaction(request: $reactionRequest)
      }
      ... on Mirror {
        reaction(request: $reactionRequest)
      }
    }
    pageInfo {
      prev
      next
      totalCount
    }
  }
}