Support the default JSON Schema keyword

[sindresorhus]

Would be nice to be able to specify default values for the rule options in the schema.

See: https://github.com/epoberezkin/ajv#assigning-defaults

I think all you have to do is to set the useDefaults: true option in

eslint/lib/util/ajv.js

Lines 18 to 23 in 2af9446

	const ajv = new Ajv({
	meta: false,
	validateSchema: false,
	missingRefs: "ignore",
	verbose: true
	});

[platinumazure]

I would be wary of potential side-effects for users. Schema validation is already pretty tricky in some ways.

If you're strongly interested in this change, would you mind seeing if you can check the effect this would have on some downstream repositories? We have the eslint-canary tool that could be used for this purpose. Thanks!

[not-an-aardvark]

@platinumazure If I'm understanding correctly, this would only affect users that are currently using a (non-functional) default key. Were those the users you had in mind, or is there something I'm missing?

[platinumazure]

@not-an-aardvark Yes; but in particular I would also like to know if there's any part of the JSON Schema grammar where "default" used to mean a property or value of "default", but would now be a metadata property indicating a default value for the object/property described by the schema.

[platinumazure]

@sindresorhus Sorry for letting this slide. Could you please provide an example of how this would benefit end users or plugin developers? E.g., would it somehow allow options to be automatically hydrated?

---

Also, going back to my earlier comments, I've thought about this some more and I think I remember that property names such as "default" tend to be in properties or elements objects within an object or array schema definition-- that is, default-as-a-keyword should be outside of those elements so there should be no collision. So I'm less concerned about a breaking change possibility here.

[ilyavolodin]

@platinumazure This would potentially benefit us as well as integrations developers. For example, currently for semi rule, always is a default option. It's not marked anywhere as a default, and the only way to figure this out is either through documentation, or by reading the code. However, if we were to use default property, you could also figure that out through schema. More than that, we could write generic code that would run before the rule executes that would set all of the defaults up, if configuration doesn't specify anything. That way, we could possibly remove a bunch of code from our rules that handles setting defaults now.

[platinumazure]

Thanks @ilyavolodin for that explanation-- I think I'm sold.

(That said, this needs TSC review anyway. I'll work on getting this on the agenda.)

[platinumazure]

TSC Summary:

JSON Schema supports a "default" keyword which allows schema authors to specify default values for optional properties in an object schema (among other cases). By allowing schemas to have defaults (a 1-line change in the ajv wrapper code), both core rules and custom rules could have default values in schemas.

Additionally (although maybe as a separate proposal), we could write some generic code when creating rule instances which would hydrate default values for any rule options not provided by the user, which would allow us to remove some code in all rules related to option defaults. There shouldn't be any impact to users because the default keyword only has its semantic meaning as a keyword outside of property lists (i.e., rules with a "default" named option should not be affected).

TSC Question: Should we enable use of "default" keyword in rule schemas?

[platinumazure]

This issue was accepted in the 2018/09/13 TSC meeting.

(Edit: Also, I noted that ajv's validate method does mutate the argument passed in once useDefaults: true is enabled. So we'll want to make sure we're defensively cloning user options or otherwise avoiding problems as best we can)

[platinumazure]

I've started working on this. I have the basic AJV changes in a branch.

Tasks remaining:

1. Add at least some defaults to core ESLint rules
2. Ensure that user-supplied configurations are not mutated by AJV (via defensive cloning where needed)

If anyone wants to try the branch for local testing, it can be found here: https://github.com/eslint/eslint/tree/json-schema-default

[nzakas]

@platinumazure are you still working on this?

[platinumazure]

@nzakas Yes, though I've been busy.

[g-plane]

@platinumazure Do you mind I help you? If so, can I push commits to json-schema-default branch directly?

[platinumazure]

@g-plane Please feel free to help and please feel free to push directly. Thanks!