Why is my ES6 webpack bundle larger when using default imports instead of named imports?

Question

I am using OpenLayers 6 and I import parts of the library using this notation:

import { Map, View } from 'ol';
import { Vector as VectorSource } from 'ol/source';
import { Vector as VectorLayer } from 'ol/layer';
// More in other files [...]

When running npm run dev I get a 9MB file for my project.

For testing purpose, I tried to replace these named imports by default imports:

import Map from 'ol/Map';
import View from 'ol/View';
import VectorSource from 'ol/source/Vector';
import VectorLayer from 'ol/layer/Vector';

Surprisingly, it reduced my bundled file to 6MB! It's 33% lighter, why is that? Shouldn't named imports be importing only required parts of modules?

EDIT 1

Following @Bergi comment, the library is available here. I use the last version which is installed through npm : v6.4.2

EDIT 2

As pointed out by @felixmosh answer, running npm run prod seems to reduce the size difference. I get a difference of 1KB from 886KB to 885KB.

Answer 1

Tree shaking, is part of the minification process. In dev bundles this process is not applied.

Try to run in "production" mode, and compare the results.

Answer 2

No, it shouldn't. It very depends on how internal libs are organized.

• If it uses require inside, the lib will not be shaked
• If it uses import * inside and uses this package, the whole * will be included
• Even! if it uses import {name} from './names' it still might not be tree-shaked

There is a good starting article about how tree shaking works and how to help it: https://webpack.js.org/guides/tree-shaking/