bundle size worse than preset-env with esmodules:true ?

I was looking into migrating a project that I maintain to preset-modules. However, I’ve noticed that the resulting bundle size is actually bigger than when I use preset-env with esmodules:true.

I’m not entirely sure that this is the fault of preset-modules, but I can’t seem to figure out why it’s behaving the way that it is.

To reproduce:

If you clone single-spa, checkout the 5.0 branch, yarn install, and then run yarn build:analyze, here’s the output (which is using preset-env with targets:{esmodules:true} - focusing only on the esm bundle:

./src/single-spa.js → ./lib/esm/single-spa.dev.js...
-----------------------------
Rollup File Analysis
-----------------------------
bundle size:    42.974 KB
original size:  53.199 KB
code reduction: 19.22 %
module count:   23

Then, if you yarn add -D @babel/preset-modules, change the .babelrc file to use @babel/preset-modules (for the “esm” environment), and then run yarn build:analyze again, here’s the result:

./src/single-spa.js → ./lib/esm/single-spa.dev.js...
-----------------------------
Rollup File Analysis
-----------------------------
bundle size:    45.098 KB
original size:  96.12 KB
code reduction: 53.08 %
module count:   24

If I do a comparison between the two compiled files, it appears that the size difference comes down to two things: 1)there’s an additional function added:

function _typeof(obj) {
  if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") {
    _typeof = function (obj) {
      return typeof obj;
    };
  } else {
    _typeof = function (obj) {
      return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj;
    };
  }

  return _typeof(obj);
}

and 2) all the arrow functions were compiled back to normal function s.

I’m not sure why either of these things are happening, especially the transpilation of arrow functions to normal functions.

[frehner]

confirmed to work while using latest babel and "bugfixes": true

[frehner]

I finally got a minute to look at this more closely, and I realized what the problem is: you’re using both @babel/preset-env and @babel/preset-modules. Babel overrides, such as the "esm" one you’re using, do not replace plugins and presets wholesale - they get merged. Since @babel/preset-env and @babel/preset-modules are different preset names, they are both included in the resolved configuration.

This will sort itself out once babel/babel#11083 is merged - you’ll be able to simply have your "esm" override set { "target": { "esmodules": true }, "bugfixes": true } and it’ll apply preset-modules optimizations.

Ah, thanks, I wasn’t aware that babel behaved that way.

Thank you for looking into this.