The problem

vscode-json-languageservice provides a UMD bundle as a main entry point. Additionally it has a faux ESM entry point using the "module" field in package.json.

This can cause various issues. For example, when bundling with esbuild:

$ esbuild vscode-json-languageservice --bundle --platform=node --outfile=ls.js 

  ls.js  5.3kb

⚡ Done in 2ms
$ node ls.js                               
node:internal/modules/cjs/loader:1080
  throw err;
  ^

Error: Cannot find module './services/jsonCompletion'
Require stack:
- ls.js
    at Module._resolveFilename (node:internal/modules/cjs/loader:1077:15)
    at Module._load (node:internal/modules/cjs/loader:922:27)
    at Module.require (node:internal/modules/cjs/loader:1143:19)
    at require (node:internal/modules/cjs/helpers:110:18)
    at /home/remco/Projects/mdx-analyzer/ls.js:30:26
    at /home/remco/Projects/mdx-analyzer/ls.js:20:13
    at Object.<anonymous> (ls.js:26:3)
    at Module._compile (node:internal/modules/cjs/loader:1256:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1310:10)
    at Module.load (node:internal/modules/cjs/loader:1119:32) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [ 'ls.js' ]
}

Node.js v18.17.0

The workaround is typically to resolve to the faux ESM files instead. So people need to write custom resolvers to use this package. As a result, you can’t really have vscode-json-languageservice as a dependency. Having it as a dependency means that package’s users would have to deal with the issue. This means vscode-json-languageservice must be bundled by consuming packages, which means it can’t be deduplicated.

Possible solutions

Switch to native ESM only

This simplifies the build process. It makes it straight-forward to consume the package. It’s supported out of the box by TypeScript. This means downstream projects also need to use ESM to use this project.

Dual publish as CJS + native ESM

Bundlers are typically pretty good at handling CJS (not UMD). Also tools properly support ESM nowadays. This means you’ll need relatively complex package.json "exports" in order to support both CJS and ESM. Also you need type definitions for both CJS and ESM.

This issue actually affects several packages, including but not limited to:

• jsonc-parser
• vscode-json-languageservice
• vscode-languageserver-types
• vscode-uri