JSON Schema – require all properties

You can just use the “minProperties” property instead of explicity naming all the fields.

{
    "type": "object",
    "properties": {
        "elephant": {"type": "string"},
        "giraffe": {"type": "string"},
        "polarBear": {"type": "string"}
    },
    "additionalProperties": false,
    "minProperties": 3
}