Are unreferenced default parameters automatically tree-shaken out by webpack?

Question

Obligatory apologies if this has been asked before, but I've been unable to find an answer online or in the webpack docs related to tree shaking.

How does tree-shaking work with default parameters? For context: I'm specifically concerned with how the webpack implementation of tree-shaking handles default parameters.

Given the following hypothetical application structure:

/
|_ dependency/
| |
| |_ dependency.js
| |_ moderatelyLargeFile.js
|
|
|_src/
  |
  |_ application.js
  |_ someSmallFile.js

and the following code in each file:

dependency.js

import moderatelyLargeFile from './moderatelyLargeFile.js';
import { someParsingFunction1, someParsingFunction2 } from 'somewhere-else-dependency';

export default function parseSomeFile(file = moderatelylargeFile) {
  let parsedResult = { ...someParsingFunction1(file), ...someParsingFunction2(file) };
  return parsedResult;
}

application.js

import parseSomeFile from './dependency/dependency.js';
import someSmallFile from './someSmallFile.js'
let result = parseSomeFile(someSmallFile);
// Do other application stuff with result

Would moderatelyLargeFile.js ever be bundled into the webpack build of application.js? Is webpack smart enough to know that the default parameter is never referenced because the single invocation provides that parameter? Or does the moderatelyLargeFile.js always get bundled regardless because it is part of the function definition?

Answer 1

No – as of April 2022, dead-code elimination of unused default parameters is not currently supported in any known bundlers.

I put together a repo with examples for the major bundlers, it tests how various bundlers handle tree shaking/dead-code elimination of Libauth's WebAssembly crypto implementations. (A good real-world example where some bundles could save hundreds of KBs.)

I also opened issues for the bundlers I'm familiar with:

• https://github.com/evanw/esbuild/issues/2185
• https://github.com/parcel-bundler/parcel/issues/7961
• https://github.com/rollup/rollup/issues/4466
• https://github.com/webpack/webpack/issues/15671

I'll try to update this answer when various bundlers add support. That repo also has a table.