Misc scribbles

You may not need a bundler for your NPM library


I have seen a couple threads on twitter where people complain about the difficulty with publishing NPM libraries or ask what starter kit they should use (or, people recommended starter packs anyways)

Example threads

One thing that is notable to me in these threads is that people often recommend that you use a bundler (a program that combines multiple src files into a single or fewer output files) when developing a library

Examples of starter packs suggested in these threads that use bundlers

Not using bundlers

In summary 2/15 do not use a bundler, 13/15 do use a bundler. Sidenote: webpack notably absent. Sidenote 2: I don't necessarily give praise to the two not using bundlers, just saying they exist.

#Why would you NOT want a bundler for your library?

My main argument is that the consumer of your library is the one that should use a bundler if it is relevant to them. If the library uses a bundler:

An example where it can actually create limitations, you might consider code splitting with async import(). If you create a single file bundle, then the consumer of your library may not be able to do code splitting properly via async import()

Another example is the now common usage of 'use client' in source code. A library targeted bundler (e.g. rollup) can't reasonably bundle this properly and prints error messages (see https://github.com/rollup/rollup/issues/4699)

#Why would you MAYBE want a bundler for your library

If you really care about producing a UMD bundle that can be used in a script tag, maybe you want a bundler, but the future does not seem to be in UMD. One other possible bundle type is maybe you like the idea of a single file ESM module. It is similar where you could maybe reference this from a script tag with type module, but this seems like a niche usage. For example, you would still have to consider:

Add-on: Another concern brought up by users in discussion thread: There is a cost to having many small files, e.g. in app startup cost on serverless or any nodejs application to loading many small files off disk. To me, this is an app level concern, similar to bundling for the browser though.

#My suggestion: no bundler, no starter pack, just tsc

I'd recommend just compiling your code with tsc, no bundler involved. This way, you can develop with typescript, it will output js files, and you can directly deploy a dist folder of js files to NPM.

#Example package.json

  "name": "yourlib",
  "version": "1.0.0",
  "main": "dist/index.js",
  "scripts": {
    "clean": "rimraf dist",
    "prebuild": "npm run clean",
    "build": "tsc",
    "preversion": "npm run build",
    "postversion": "git push --follow-tags"
  "files": ["dist", "src"],
  "devDependencies": {
    "rimraf": "^3.0.2",
    "typescript": "^4.6.2"

#Features of the above package.json

#Example tsconfig.json

  "include": ["src"],
  "compilerOptions": {
    "target": "es2018",
    "outDir": "dist",
    "lib": ["dom", "esnext"],
    "declaration": true,
    "moduleResolution": "node",
    "sourceMap": true,
    "strict": true,
    "esModuleInterop": true

#Features of the above tsconfig.json

#What about testing?

Adding testing is not immediately solved by the above, but bundling doesn't really help testing anyways. It's just a starter pack feature we can add on. Some options you have include


It is tempting to have nice zero-config solutions and starter kits, but to me, it is not really beneficial to use the bundler aspect of many of these for publishing to NPM. Am I wrong? Let me know if I am.

Also, these starter kits may not be maintained for perpetuity. Our team used tsdx for some time, but it was not maintained well, and used old typescript version 3.x, and it ended up being hard to remove from our codebase. Learning the basic tools like tsc will help

#Footnote 1: Shipping "pure-ESM"

Do you want to make a pure-ESM package? Then you do not want to use "moduleResolution": "node" in tsconfig.json, and you will want to set "type": "module" in package.json.

You may also need to explicitly import with .js extensions in your source code, even if you write .ts.

This is awkward, and something the community is still grappling with. This is ACTUALLY one of the areas that a bundler can help, because by bundling, you don't ever encounter any actual imported files in the published artifacts.

You will also probably use the "exports" field in package.json.

If you have ever stumbled on this topic, you will probably want to see this link https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c

#Footnote 2: Shipping ESM and CommonJS side by side without going "pure-ESM"

It can be tricky to go pure-ESM, but you can go most of the way there by using both the "main" and "module" fields in package.json

The "module" field is a field only bundlers recognizes https://stackoverflow.com/questions/42708484/what-is-the-module-package-json-field-for

Note: This is different from the "type":"module" which marks your module as pure-ESM!

I have found this technique can go a long ways towards keeping your package compatible with nodeJS and bundlers and it does not require "export maps" or anything which I have found to be difficult to configure

#Footnote 3: What does it look like when you compile with tsc?

When I refer to compiling with tsc above, I compile a src directory into a dist directory

So if I have:


Running tsc will output:


Then, the dist and src directories are published to npm which enables the sourceMaps to work.

Note: We do not need to explicitly say where the typescript types are with "types" in package.json, many starter packs do this but it is unneeded for this package as the d.ts files are automatically found.

See https://cmdcolin.github.io/posts/2021-12-31-npm-package for my article on creating a typescript package for npm

#Footnote 4: Publishing the raw JS with JSDoc checked types

You may not even need tsc to compile your dist folder. You can literally publish your source .js files as-is to NPM. This suggestion comes from @trevmanz 1 2. I don't personally use this technique yet but there are many users with this workflow

An interesting thing is you can write in .js but still get typescript to get type checking using jsdoc, just use allowJs/checkJs flags in tsconfig.json

#Footnote 5: Using JSX does not require bundler

What about using React in your library? Bundlers are still not needed, and not even babel is needed: you can code your library as in jsx or tsx files and use tsc to compile it, and it will be converted to React.createElement statements (or the newer jsx transform if you elect to use it in your tsconfig).