JSONPath Plus feature existence fails in Stoplight Spectral but works in JSONPath Plus

[Laavanja19]

Describe the bug
Consider the given JSONPath expression:

$.store.book[?(@property)]

When this is used inside a Spectral ruleset, Spectral throws an error.

Error #1: '@property' is not defined

However, @property is a valid JSONPath Plus feature and in JSONPath Plus this expression does not throw an error. Instead, it returns all the elements whose property is truthy.

To make this work in Spectral, the above JSONPath expression must be written as a function

$.store.book[?(@property())]

Here, book is an array, so the property value for each array element corresponds to its array index. However in JavaScript, the condition if(0) evaluates to false.

Because, JSONPath Plus features are implemented in JavaScript, the first element of the book array has a property value of 0, which is considered falsy. As a result,

• First array element is excluded.
• Only the remaining array elements are returned.
  But the expected behavior is that all array elements should be returned, because each element does have a property (its index), and the filter is meant to check for the existence of a property—not its truthiness.

To Reproduce

1. JSON Document

{
    "store": {
        "book": [{
            "category": "reference",
            "author": "Nigel Rees",
            "title": "Sayings of the Century",
            "price": 8.95
        },
        {
            "category": "fiction",
            "author": "Evelyn Waugh",
            "title": "Sword of Honour",
            "price": 12.99
        },
        {
            "category": "fiction",
            "author": "Herman Melville",
            "title": "Moby Dick",
            "isbn": "0-553-21311-3",
            "price": 8.99
        },
        {
            "category": "fiction",
            "author": "J. R. R. Tolkien",
            "title": "The Lord of the Rings",
            "isbn": "0-395-19395-8",
            "price": 22.99
        }],
        "bicycle": {
            "color": "red",
            "price": 19.95
        }
    }
}

2. Ruleset

rules:
  price-not-greater-than-20:
    description: Price must not be greater than 20
    message: "The price '{{value}}' is greater than 20"
    severity: error
    given: "$.store.book[?(@property())]"
    then:
      function: falsy

3. Run this CLI command

spectral lint data.json --ruleset ruleset.yaml

4.Actual result

 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[1]
 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[2]
 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[3]

4. Expected result

 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[0]
 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[1]
 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[2]
 error  price-not-greater-than-20  The price '[object Object]' is greater than 20  store.book[3]

Environment (remove any that are not applicable):

• Spectral Version : 6.15.0
• OS: [Ubuntu (Linux)]