tree-shake-test

Setup:

cd importee
npm link

cd ../importer
npm link importee
npm i

Webpack:

npm run build-webpack

You should have a minified bundle-webpack.js file at importer/output that only includes Person. It will look something like this at the bottom of the file:

function(e, t, n) {
    "use strict";
    t.a = class {
        constructor() {
            console.log("[Person]")
        }

        sayHello() {
            console.log("Hello")
        }
    }
}

This is because we've declared named exports in our entry point file (importee/src/index.js):

export { default as Person } from './person';
export { default as Dog } from './dog';

Comment out those exports and uncomment all the other lines:

// export { default as Person } from './person';
// export { default as Dog } from './dog';
 
import Person from './person';
import Dog from './dog';
 
export default {
    Person: Person,
    Dog: Dog
};

Run npm run build again and check importer/output/bundle-webpack.js. Despite our importing project only specifying that it wanted Person, it has included both classes exported by importee - Person AND Dog:

function(o, e, t) {
    "use strict";
    e.a = class {
        constructor() {
            console.log("[Person]")
        }

        sayHello() {
            console.log("Hello")
        }
    }
}, function(o, e, t) {
    "use strict";
    e.a = class {
        constructor() {
            console.log("[Dog]")
        }

        bark() {
            console.log("Bark!")
        }
    }
}

This is down to importee now exporting a default object, which includes all of its internal modules. Bundling tools don't tree shake this, as it's one export. It's implicit that if you are a library author and are declaring a default export, you expect the consumer to import EVERYTHING, whether they use it all or not.

Rollup:

Go back and change importee/src/index.js to:

export { default as Person } from './person';
export { default as Dog } from './dog';

Run:

npm run build-rollup

Open bundle-rollup.js at importer/output. Did you expect Rollup to tree shake Dog out of your bundle? Well it didn't! As it states in the Rollup documentation:

...Rollup has to be conservative about what code it removes in order to guarantee that the end result will run correctly. If an imported module appears to have side-effects, either on bits of the module that you're using or on the global environment, Rollup plays it safe and includes those side-effects.

So Rollup is erring on the side of caution, and including Dog as well.