Add ESM 'module' entrypoint

[FredKSchott]

Pulled off from: https://www.pika.dev/npm/snowpack/discuss/71

Hey @bcherny, thanks for the great open-source library. A Snowpack user was having some trouble building your package, and I thought I'd loop you in. Since Snowpack is powered by Rollup, there's a good chance that all Rollup users are having trouble with undux due to circular references in your "main" Common.js (CJS) entrypoint.

Circular dependency: node_modules/undux/dist/src/index.js -> node_modules/undux/dist/src/react/index.js -> node_modules/undux/dist/src/react/connect.js -> /Users/crabasa/Code/fibonacci/node_modules/undux/dist/src/index.js?commonjs-proxy -> node_modules/undux/dist/src/index.js
Circular dependency: node_modules/undux/dist/src/index.js -> node_modules/undux/dist/src/react/index.js -> node_modules/undux/dist/src/react/createConnectedStore.js -> node_modules/undux/dist/src/index.js
Circular dependency: node_modules/undux/dist/src/index.js -> node_modules/undux/dist/src/react/index.js -> node_modules/undux/dist/src/react/createConnectedStoreAs.js -> node_modules/undux/dist/src/index.js
Error: '__moduleExports' is not exported by node_modules/undux/dist/src/index.js
    at error (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:5400:30)
    at Module.error (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:9820:16)
    at handleMissingExport (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:9721:28)
    at Module.traceVariable (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:10159:24)
    at ModuleScope.findVariable (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:8766:39)
    at Identifier$1.bind (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:4403:40)
    at ExportDefaultDeclaration.bind (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:3152:23)
    at Program$1.bind (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:3148:31)
    at Module.bindReferences (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:9792:18)
    at Graph.link (/Users/crabasa/Code/fibonacci/node_modules/rollup/dist/shared/node-entry.js:13425:20)

Since bundlers are better at code-splitting/tree-shaking ESM than CJS, and since your only consumers today are using this library through bundlers, have you thought at all about adding a "module" entrypoint to your package.json so that your users get better-optimized bundles? "module" is the entrypoint that 90,000+ packages use on npm to expose their ESM entrypoint (we track this on pika.dev).

I saw that you have a "main:esnext" build, but since TypeScript's esnext target can include experimental JavaScript, this may not be the best choice for a library. target: es2020 would be better only so that bundlers would be guaranteed to understand the code and not break on any experimental (not yet ratified as ES20XX) bits.

There are a few ways to go about this. How would you feel about a PR that changed the tsconfig.esnext.json "target" property from "esnext" to "es2020', and then updated your package.json to add a "module" property pointing to that build? That way this library would be less likely to break Snowpack & Rollup users, and all users bundlers would benefit from a faster ESM library in production.

[bcherny]

Sounds reasonable. Happy to stamp a PR that does that.

[FredKSchott]

Awesome, created a PR at #105 👍