Modules, Runtime & Tooling

Transpilation Targets & Polyfills

Transpilation rewrites syntax for older runtimes; polyfills provide missing runtime APIs. They solve different compatibility problems and are easy to confuse.

Transpilation rewrites syntax
Polyfills provide APIs
Type declarations are not runtime support
Targets should reflect supported environments
Polyfills affect global behavior

Compatibility tools
Tool	Solves	Does not solve
Transpilation	Unsupported syntax	Missing built-in APIs
Polyfill	Missing global/runtime API	Syntax parsing errors
Ponyfill	Local implementation	Global compatibility expectations
Browserslist/target	Environment selection	Actual user measurement
tsconfig lib	Compile-time API types	Runtime support

Syntax vs API

// Syntax can be transpiled:
const name = user?.profile?.name ?? "Unknown"

// Missing API needs runtime support or polyfill:
const copy = structuredClone(value)

The first is syntax; the second is a runtime API.