Dead-code elimination of unused default parameters

[bitjson]

Feature Use Case

I maintain Libauth, a library that offers WebAssembly crypto implementations (ripemd160, sha1, sha256, sha512, and secp256k1). As a pure ESM library, Libauth can asynchronously instantiate each WASM implementation internally, exporting simple interfaces that behave like collections of JS-only functions (with better performance). Many of Libauth's exported functions also use one of these built-in WASM instances as a default parameter. For example, decodeBase58Address has the definition:

export const decodeBase58Address = (
  address: string,
  sha256: { hash: (input: Uint8Array) => Uint8Array } = internalSha256
) => {
  // ...
};

Most applications can simply call decodeBase58Address(address) to automatically use the default, WASM-based sha256 implementation (internalSha256).

However, applications that already have another sha256 implementation can provide that implementation as the second parameter: decodeBase58Address(address, mySha256Implementation). In this case, the default parameter (internalSha256) is dead code and should be possible to eliminate from the application's bundle, saving hundreds of KBs from those bundles.

Feature Proposal

During dead-code elimination/tree shaking, default parameters that aren't used by the bundle can be removed.

For this simple example:

module.js:

const internalAdd = (a, b) => (a === '0' ? b : a + b); // should not appear in bundle
export const addAndLog = (a, b, impl = internalAdd) => console.log(impl(a, b));

app.js:

import { addAndLog } from './module.js';
const myAdd = (a, b) => a + b;
addAndLog(1, 2, myAdd); // => 3

The produced bundle should include only:

const addAndLog = (a, b, impl) => console.log(impl(a, b));
const myAdd = (a, b) => a + b;
addAndLog(1, 2, myAdd); // => 3

A detailed example (testing with all known bundlers) is available at this repo: https://github.com/bitjson/shake-default-params

[lukastaegert]

Hi, sorry for the silence. I think this is actually an interesting idea and while it is quite non-trivial, I did some work in #4498 which should make this possible. You can already try this out, there are just some small things I want to polish.

[lukastaegert]

#4498 is ready now, you can check in the REPL how it handles your example https://rollupjs.org/repl/?pr=4498&shareable=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

[bitjson]

@lukastaegert wow, fantastic! Thanks so much for working on this! Looking forward to seeing the PR land 👍