how to check the given field is missing from the given object?

[Yonokawa]

$ cat x.json
{"a": 123}

$ ajson 'is_null($.b)' x.json
null

$ ajson '$.b == null' x.json
[]

$ ajson '$' x.json
[{"a": 123}]

the go code works the same way, if the given key doesn't exist, its value is not null, and is_null() check is false.

so is there any way to check this? or a new function helper is needed?

[spyzhov]

Hi! Right now there is no easy way how to do it, this possibility will be unblocked with the function of many arguments.

I can suggest you to use this way: https://play.golang.com/p/HRFPdZPCxB-

package main

import (
	"fmt"

	"github.com/spyzhov/ajson"
)

func main() {
	json := []byte(example)

	root := ajson.Must(ajson.Unmarshal(json))
	exists := must(root.JSONPath(`$.store.book[?(@.price || @.price == false || @.price == 0 || is_null(@.price))]`))
	NOT_exists := must(root.JSONPath(`$.store.book[?(not(@.price || @.price == false || @.price == 0 || is_null(@.price)))]`))

	fmt.Println("Exists:")
	for _, node := range exists {
		fmt.Printf(
			"  > %s - %v\n",
			node.MustKey("title").String(),
			must(node.MustKey("price").Value()),
		)
	}
	fmt.Println("NOT Exists:")
	for _, node := range NOT_exists {
		fmt.Printf("  > %s", node.MustKey("title").String())
	}
}

func try(err error) {
	if err != nil {
		panic(err)
	}
}

func must[T any](v T, err error) T {
	try(err)
	return v
}

const example = `
{
  "store": {
    "book": [
      {
        "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century"
      },
      {
        "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 0
      },
      {
        "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": null
      },
      {
        "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": false
      }
    ]
  }
}`

Output:

Exists:
  > "Sword of Honour" - 0
  > "Moby Dick" - <nil>
  > "The Lord of the Rings" - false
NOT Exists:
  > "Sayings of the Century"

And so, in your case, I would suggest use:

$> ajson '(@.a || @.a == false || @.a == 0 || is_null(@.a))' x.json
true

$> ajson '(@.b || @.b == false || @.b == 0 || is_null(@.b))' x.json
false

$> ajson 'not(@.b || @.b == false || @.b == 0 || is_null(@.b))' x.json
true