Building a JSON Schema-enabled `JSON5` mode for codemirror, monaco and/or graphiql

4 min read

There are several potential routes for providing JSON5 variables support for monaco-graphql and graphiql, one of which we are already pursuing!

This article is also useful for anyone building json4 or json5 enabled feature modes for any IDE.


monaco-graphql uses a getVariablesJSONSchema utility I wrote, that supplies a json schema to monaco-json for variables completion. It accepts either an AST or string of the operation, and returns a complete JSON description to validate/hover/autocomplete these input types and other scalars.

This leverages monaco-json's api which allows you to define a set of json schemas for these language features, which utilizes the underlying vscode-json-languageservice and their custom jsonParser. you have likely seen this in action on codesandbox, or in vscode when defining json configuration files like package.json or tsconfig.json.

Key features of a json-schema enabled JSON4/JSON5 mode:

1. Continues working when string parse methods fail, such as JSON5.parse() or JSON.parse()

JSON/5 language features are still needed when you have a SyntaxError

We created a utility you can see in the PR below using cm6's syntax tree that still parses and aggregates json pointers and positional data when the string parse fails.

This allows you to get completion, hover, and validation data when typing out JSON4 or JSON5 properties, regardless of whether the string is valid. we have similar fault tolerance for graphql-language-service's parser.

This is why you can still use features like completion and hover for operation documents when graphql.parse() would fail.

2. Hyper-positionally aware

Another reason you can't build full modes with JSON5.parse() or JSON.parse() besides fault tolerance is the lack of positional information. The errors give you a little bit, but not the entire range you need for editor tooling.

json-source-map gives positional information, but lacks fault tolerance, unlike the parseJSONDocument library we built for codemirror 6, that uses the cm6 syntax tree. This library is often seen along with ajv for codemirror6 json schema implementations, but the lack of fault tolerance and the performance issues are immediately apparent.

3. Dynamic types aren't needed for graphql variables (yet)

For graphql variables at least, it doesn't need to worry about oneOf, anyOf or other dynamic types in the json-schema spec yet. Polymorphic input features for graphql are still in progress.

These would still need to exist for a complete, multi-purpose JSON Schema mode, outside of graphql variables applications.

4. Language Feature Complete

it should provide what monaco-json provides already for JSON4, and thus we already provide via monaco-graphql - validation, hover, autocompletion, and formatting

Proposed approaches:

A. CM6 JSON5 Variables Editor Plugin!

@imolorhe (opens in a new tab), GitHub Copilot and I are making a lot of headway on a json (JSON4) json-schema-enabled mode for cm6! And it now supports json5! (opens in a new tab)

The cm6 syntax tree is a breeze to work with and very performant, and would be easy to translate to JSON5 using their existing JSON5 mode (which just calls JSON5.parse() and handles validation messages for now, no JSON Schema in scope). Update: We've now implemented it!

So, as we do with cm6-language-json-schema and @codemirror/lang-json, we can extend the existing codemirror-json5 mode that exists to provide JSON Schema support for json5 using their syntax tree, and then provide a cm6 JSON5 variables editor as a graphiql plugin in place of the monaco-json editor.

There are vscode-style keymaps and other ways cm6 could be extended to have json5 enabled feature parity with our current monaco-json implementation.

this would be used alongside monaco-graphql. codemirror 6 is extremely lightweight and the display is easily customizeable, and it wouldn't be a problem to use the editor runtimes together, especially via a plugin for overriding the variables editor, or a plugin for providing multiple "variables mode" options.

B. Extend monaco-json and vscode-json-languageservice to Support JSON5

The vscode-json-languageservice that belies monaco-json would need either a seperate JSON5 parser, or would need to be improved to handle all of the rules of the JSON5 spec. currently, it uses config options to support (most of) CSON, but assumes all property names are double quoted, etc. again, JSON5.parse() cannot be used for this because it needs to parse incomplete and invalid documents to be a useful parser for IDE purposes.

This would also be determined by microsoft's willingness to pursue such a goal. It would be very powerful, modernize their ecosystem, and give 80+ percent of developers the ability to use package.JSON5, tsconfig.JSON5, seamlessly!

C. Create a monaco-json5?

Someone could also create a new monaco-json style-like mode from scratch for JSON5. I would look into existing JSON5 vscode extensions, and other libraries that implement JSON5 features and validation that have json-schema support, and use this as the underlying implementation. Monaco, unlike codemirror, does not bring it's own parser and syntax tree conventions, so this can be both freeing, and a performance issue.

In fact... what if someone made a monaco-json5 language mode with.... @lezer/common and @codemirror/language??? it's technically possible!

Ask Me for Help

I love to help people to develop IDE/language editor tooling, so ping me! You can find me on the graphql discord server :)

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