ts/isolated-modules

Why

Vite (through esbuild or Oxc) compiles each file alone and never type-checks. Features that need the whole program, such as const enum or an import of a type without type, compile in tsc but break or leave stray imports in the bundle. Without the flag, the editor and tsc stay silent about them.

Fix

Add "verbatimModuleSyntax": true (which implies isolated modules and keeps imports exactly as written) or at least "isolatedModules": true, then write import type { User } from './types' for type-only imports. Run tsc --noEmit (or vue-tsc, svelte-check, astro check) in CI, since the build itself does not check types.

Examples

// bad
{ "compilerOptions": { "module": "esnext", "moduleResolution": "bundler", "strict": true } }
// good
{ "compilerOptions": { "module": "esnext", "moduleResolution": "bundler", "strict": true, "verbatimModuleSyntax": true } }

Sources

• Vite, Features (TypeScript, isolatedModules)
• TypeScript, TSConfig reference (verbatimModuleSyntax)

Read it in the terminal

npx github:dawidolko/frontend-skills explain ts/isolated-modules