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