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Using Package Exports to Define Multiple Entrypoints


Created Dec 22, 2021 – Last Updated Dec 22, 2021

JavaScriptTooling
Digital Garden

At work I was recently faced with this error and I was a bit clueless at first what it could be:

shell
ModuleParseError: Module parse failed: Unexpected character ''

A colleague mentioned that they also have seen it while doing some work on ES Modules (ESM). So what was the cause of this error and how did I solve it? TL;DR: Use exports in package.json.

#Setup

So in short, the setup was as following:

  • Package frontend imported a helper function from package utils. frontend is used in client-side code, e.g. React
  • Package utils contains a couple of helper functions, both with and without 3rd-party dependencies. It is written in TypeScript and the output is compiled into a dist folder at the root of the package.
  • webpack is used for frontend to bundle all code

The file inside frontend imports it like this:

frontend/src/link.js
js
import { convertPath } from "utils"
export const linkHelper = (input) => {
return convertPath(input)
}

The utils package contains multiple files, for example two files:

utils/src/convert-path.ts
ts
export const convertPath = (path: string) => path.toUpperCase()
utils/src/watch.ts
ts
import * as path from "path"
import * as chokidar from "chokidar"
export const getDirname = (path: string) => path.dirname(path)
export const watchDir = (path: string, glob: string) => {
return new Promise((resolve) => {
chokidar.watch(glob, { cwd: path }).on("ready", () => resolve())
})
}

Those get exported in an index.ts file:

utils/src/index.ts
ts
export { convertPath } from "./path"
export { getDirname, watchDir } from "./watch"

And referenced in the main field of package.json with "main": "dist/index.js".

#Solution

The problem with this setup is that webpack (or probably any other bundler) has problems tree-shaking the Node.js and chokidar bits from utils when you only import convertPath in the frontend package. This is why one sees the error mentioned at the beginning.

But ESM and/or exports in package.json can come to the rescue here! Node.js itself has extensive documentation about Package entry points so for the sake of this small post here’s just the solution on how to fix the problem for the given example.

First, add the exports to utils:

utils/package.json
json
"exports": {
".": "./dist/index.js",
"./*": "./dist/*.js"
}

This enables the change in this second step:

frontend/src/link.js
js
import { convertPath } from "utils/convert-path"
export const linkHelper = (input) => {
return convertPath(input)
}

Notice the different import for convertPath? This is a new entrypoint you can use the ignore all the stuff that you don’t need. Awesome!


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