preset-env-usebuiltins.md

Polyfillying the standard library when using babel-preset-env

• Overview of a workflow based on env: http://2ality.com/2017/07/npm-packages-via-babel.html
• env has the option useBuiltins that helps with polyfilling the standard library: https://github.com/babel/babel-preset-env#usebuiltins

Browsers

Approach for untranspiled code (meant to be transpiled by Babel and, e.g., webpack):

• No package ever polyfills.
  • Use only functionality at stage 4 or older.
• The entry point does import "babel-polyfill"; for which env.useBuiltins generates the imports appropriate for the current targets.
  • Benefit: the bundle only includes functionality that the target platforms don’t have. Think: custom-tailored babel-polyfill.

Node.js

For Node.js, the untranspiled code would be transpiled via target: { node: "current" }.

• To play it safe, should every package do import "babel-polyfill"; at its entry point?
  • Note: not as problematic, because bundle size is not a factor on Node.js.

• babel-polyfill does include regenerator-runtime: https://github.com/babel/babel/blob/master/packages/babel-polyfill/package.json
• The approach I linked to is for supporting both browsers and Node.js: http://2ality.com/2017/07/npm-packages-via-babel.html