`extension` function works inconsistently

[alexlipovka]

Consider this example data:

{
    "extension": [
        {
            "url": "find-me",
            "extension": [
                {
                    "url": "inner-1",
                    "valueString": "test-1"
                },
                {
                    "url": "inner-2",
                    "valueString": "test-2"
                }
            ]
        }
    ],
    "item": [
        {
            "text": "Weight",
            "linkId": "weight",
            "extension": [
                {
                    "valueCoding": {
                        "code": "kg",
                        "system": "http://unitsofmeasure.org",
                        "display": "kg"
                    },
                    "url": "http://hl7.org/fhir/StructureDefinition/questionnaire-unitOption",
                  "extension": [
                      {
                          "url": "inner-1",
                          "valueString": "test-1"
                      },
                      {
                          "url": "inner-2",
                          "valueString": "test-2"
                      }
                  ]
                }
            ],
            "type": "quantity"
        }
    ]
}

1. Query extension('find-me') returns []
2. Query extension.where(url='find-me') returns

- url: find-me
  extension:
    - url: inner-1
      valueString: test-1
    - url: inner-2
      valueString: test-2

3. Query extension.extension('inner-1') returns

- url: inner-1
  valueString: test-1

Expected:
query extension('find-me') returns the same as extension.where(url='find-me')

[yuriy-sedinkin]

Yes, executing the extension(url) function at the root of an arbitrary JSON object requires some modifications to the fhirpath.js.

As a workaround, try executing this function on a FHIR resource. If you want to execute this function on data previously retrieved from a resource without using fhirpath.js, you should look for the text in the README.md file regarding "fhirpath expression relative to base".