Code for Issue

index.js

const before = [
  {
    "classes": [
        "5C"
    ],
    "period": 3,
    "new-teacher": "Hans",
    "old-teacher": "Peter",
    "new-subject": {
        "short": "Mu",
        "full": "Musik"
    },
    "old-subject": {
        "short": "E",
        "full": "Englisch"
    },
    "room": "Mu2",
    "description": null
  },
  {
      "classes": [
          "5D"
      ],
      "period": 3,
      "new-teacher": "Karl",
      "old-teacher": "Gustaf",
      "new-subject": {
          "short": "M",
          "full": "Mathematik"
      },
      "old-subject": {
          "short": "M",
          "full": "Mathematik"
      },
      "room": "1.018",
      "description": null
  }
]
const after = [
    {
    "classes": [
        "5C"
    ],
    "period": 3,
    "new-teacher": "Hans",
    "old-teacher": "Peter",
    "new-subject": {
        "short": "Mu",
        "full": "Musik"
    },
    "old-subject": {
        "short": "E",
        "full": "Englisch"
    },
    "room": "Mu2",
    "description": null
  }
]

const diffs = detailedDiff(before, after)
//expected:
/*
>  {
>    added: [Object: null prototype] {},
>    deleted: [Object: null prototype] {
>      '1': undefined
>    },
>    updated: [Object: null prototype] {}
>  }
*/

//what acctually happens:
//because something from the array got removed, the indices of every entry are changing and that's registered as updated
/*

>  {
>    added: [Object: null prototype] {},
>    deleted: [Object: null prototype] {},
>    updated: [Object: null prototype] {
>      '0': [Object: null prototype] {
>        'new-teacher': 'Karl',
>        period: 3,
>        'new-subject': [Object: null prototype],
>        classes: [Object: null prototype],
>        'old-subject': [Object: null prototype],
>        description: null,
>        'old-teacher': 'Gustaf',
>        room: '1.018'
>      }
     }
>  }

*/