How do I use the if-then-else condition in json schema?

Question

A relatively new addition to JSON Schema (draft-07) adds the if, then and else keywords. I cannot work out how to use these new keywords correctly. Here is my JSON Schema so far:

{
    "type": "object",
    "properties": {
        "foo": {
            "type": "string"
        },
        "bar": {
            "type": "string"
        }
    },
    "if": {
        "properties": {
            "foo": {
                "enum": [
                    "bar"
                ]
            }
        }
    },
    "then": {
        "required": [
            "bar"
        ]
    }
}

If the foo property equals "bar", then the bar property is required. This works as expected.

However, if the foo property does not exist or input is empty then I don't want anything. How to achieve this?

Empty input:

{}

Found Errors:   
    Required properties are missing from object: bar.
    Schema path: #/then/required

I use online validation tool:

https://www.jsonschemavalidator.net/

Answer 1

The if keyword means that, if the result of the value schema passes validation, apply the then schema, otherwise apply the else schema.

Your schema didn't work because you needed to require "foo" in your if schema, otherwise an empty JSON instance would pass validation of the if schema, and therefore apply the then schema, which requires "bar".

Second, you want "propertyNames":false, which prevents having any keys in the schema, unlike if you were to set "else": false which would make anything always fail validation.

{
  "type": "object",
  "properties": {
    "foo": {
      "type": "string"
    },
    "bar": {
      "type": "string"
    }
  },
  "if": {
    "properties": {
      "foo": {
        "enum": [
          "bar"
        ]
      }
    },
    "required": [
      "foo"
    ]
  },
  "then": {
    "required": [
      "bar"
    ]
  },
  "else": false
}