Polyfills and Core-js

The polyfill vs transpile distinction

Transpilation converts syntax. Polyfills add missing functions: Promise, Array.prototype.includes(), Object.assign(), etc. Both are needed for legacy browser support.

Core-js: the standard polyfill library

core-js is the most complete polyfill library. Babel can inject only the polyfills your code actually needs via @babel/preset-env with useBuiltIns.

Configuring polyfill injection

Use useBuiltIns: 'usage' to automatically inject only needed polyfills. Reduces bundle size compared to including all of core-js.

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "useBuiltIns": "usage",
        "corejs": "3.32"
      }
    ]
  ]
}