condition on the root

[gsouf]

Hi,

I have this kind of json:

{
   foo: 'bar',
   baz: 'qux'
}

I want to read value of baz only if foo === "bar". I can get it to work for array structures, but not at the root.

I tried many things like:

$[?(@.foo === "bar")].baz
$?(@.foo === "bar").baz
$.baz[?(@parent.foo === "bar")]
etc..

It just looks like we cannot operate any filters on the root element.

[brettz9]

Yes, I believe you're right that there is no current core way to achieve this pretty fundamental filtering type (by object)--not only at root but anywhere. I think there may not have been a great need previously since programmers would tend to drill down before or afterward.

The only option you could have--if the engine is under your control (though if it were, I presume you could also add the root inside an array)--would be to use otherTypeCallback:

const {JSONPath} = require("jsonpath-plus")

const json = {
   foo: 'bar',
   baz: 'qux'
};

JSONPath({path: '$.@other()', json, otherTypeCallback (value, path, parent, parentPropertyName) {
  return value && value.foo === 'bar';
}});

Demo: https://runkit.com/brettz9/5e8c8b95dee34d001c58e0f2

[gsouf]

@brettz9 thanks for your answer! Actually I didn't give up and I tried again. And I found a way to achieve it. Not the most easy to understand when reading but at least it works:

  $[?(@property === "foo" && @ === "bar")]^.baz

We filter the property by name and value in order to get 0 or 1 (and only 1) match if the property matches. Then when go back to the parent (because we ensured to have 0 or 1 match then it will return the parent only if the propety matched). Then we just do the stuff on the parent.

I'm closing this one because it's solved for me.

[brettz9]

Ahhh--shows how much I know! :) Nice work finding the way!

I just now added a note on the README to highlight this possibility, so it is now hopefully more evident what is possible (though if not, I'm open to doc PRs).