schema.json

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "properties": {
        "val": {
            "type": "number"
        },
        "valNullable": {
            "type": [
                "null",
                "number"
            ]
        },
        "valOpt": {
            "type": "number"
        },
        "valTrue": {
            "enum": [
                true
            ],
            "type": "boolean"
        },
        "valTrueOpt": {
            "enum": [
                true
            ],
            "type": "boolean"
        },
        "valTrueOrNull": {
            "anyOf": [
                {
                    "enum": [
                        true
                    ],
                    "type": "boolean"
                },
                {
                    "type": "null"
                }
            ]
        },
        "valUndef": {
            "type": "number"
        },
        "valVoid": {
            "type": "number"
        }
    },
    "required": [
        "val",
        "valNullable",
        "valTrue",
        "valTrueOrNull"
    ],
    "type": "object"
}