[Help Wanted] How can I get descriptive error messages when the schema is invalid.

[JeelRajodiya]

I have the following schema

{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "array",
    "items": {
        "type": "string"
    }
}

and following data

[1,2,3]

Now when I validate the schema, the I get the following error

{"keyword":"https://json-schema.org/keyword/items","absoluteKeywordLocation":"http://example.com/schemas/string#/items","instanceLocation":"#","valid":false,
"errors":[{"keyword":"https://json-schema.org/evaluation/validate","absoluteKeywordLocation":"http://example.com/schemas/string#/items","instanceLocation":"#/0","valid":false,
"errors":[{"keyword":"https://json-schema.org/keyword/type","absoluteKeywordLocation":"http://example.com/schemas/string#/items/type","instanceLocation":"#/0","valid":false,"errors":[]}]}]}

My code

What I am looking for is a descriptive error similar to what cfworker/json-schema throws

Items did not match schema. Instance type "number" is invalid. Expected "string".

[jdesrosiers]

I never implemented descriptive error messaging. The theory was that the standard JSON Schema output format, would allow people to write tools to consume that format to give you nice error messaging that makes sense for your specific use case. There are a couple such implementations that use AJV's proprietary output format. There's one designed for CLI consumption and another designed for API responses. Unfortunately, no one ever built things like that for the standard output format, so for now there isn't really something I can point you to solve this problem. I'll have to give up waiting for someone else to do it and do it myself, but I haven't had time yet.

I recently shared with another user some code to get started if you want to see how to build out some messaging yourself.

[JeelRajodiya]

Thanks a lot!
Should I close the issue?

[jdesrosiers]

I think you can leave it open. It will help others with the same question find the answer they're looking for. I'll close it in the future if/when the situation changes.

[GabenGar]

The reason no one (including the AJV maintainer) has written the standard message parser is because it is a humongous task once you realize it is pretty useless without multilang support. And multilang support brings its own can of worms: bloated bundle size for client and exploding string cache for server to name a few.

[srivastava-diya]

@JeelRajodiya @jdesrosiers hi i am very new to open source and this is my first time asking a doubt i just wanted to ask if we encounter problem to get descriptive error messages in typescript only or in other languages as well because i feel python can handle descriptive error messages pretty well.

[jdesrosiers]

@srivastava-diya It can be done in any programming language. I gave two examples of where something similar has been implemented in JavaScript/TypeScript in my first comment. The only problem is that those libraries use AJV's proprietary output format as input, so they don't work with this library that uses the standard output format. What's needed is for someone to adapt one of those packages for a different input or write something to adapt the standard output to match AJV's output format.

[valerii15298]

In case anyone is curious:
Here is the output structure described in the spec: https://json-schema.org/draft/2020-12/json-schema-core#name-output-structure
And schema of the output: https://json-schema.org/draft/2020-12/output/schema

It is really a shame that after quite a few years no one(or almost no one except this library) implemented standard output format as specified by the spec... Even without multilang support it would be quite useful.

I also noticed that they have error property for each error which in the examples seems like a human readable error(but it is optional)

[jdesrosiers]

FYI, this summer I had someone working on an implementation for Google Summer of Code. It's left unfinished for now, but it's a start if anyone is interested in contributing.

https://github.com/hyperjump-io/better-json-schema-errors

[srivastava-diya]

FYI, this summer I had someone working on an implementation for Google Summer of Code. It's left unfinished for now, but it's a start if anyone is interested in contributing.

https://github.com/hyperjump-io/better-json-schema-errors

hey @jdesrosiers i would love to work on this implementation. How can i start?

[jdesrosiers]

@srivastava-diya I'd be happy for the help. The first step is to read the documentation for that repo and get familiar with the code. Identify where there are gaps in the implementation or things that can be improved and make a proposal for what you want to do. We'll discuss it and when I give approval, you can implement your proposal and make a PR. Then make another proposal and repeat as much as you're willing.