vitejs/vite · error · Error

Name in package.json is required if option…

Error message

Name in package.json is required if option "build.lib.fileName" is not provided.

What it means

When building a library, Vite resolves the output filename from build.lib.fileName, the package.json name field, or the entry key name. If fileName is omitted AND the entry is a single string AND package.json has no name, resolveLibFilename has nothing to base the filename on and throws. The check lives in resolveLibFilename, called while generating per-format output names.

Solutions

1. Add a "name" field to the nearest package.json (e.g. "name": "my-lib").
2. Alternatively, set build.lib.fileName explicitly: fileName: 'my-lib'.
3. If the entry is an object (multiple entries), the entry key is used as the fallback name, so switch to object-form entry.

Example fix

// before (vite.config.js)
build: { lib: { entry: 'src/index.ts', formats: ['es'] } }
// after
build: { lib: { entry: 'src/index.ts', fileName: 'my-lib', formats: ['es'] } }
// or in package.json: { "name": "my-lib" }

const fs = require('fs');
function ensureLibNameOrFileName(libOptions, root) {
  if (libOptions?.fileName) return true;
  if (libOptions && typeof libOptions.entry === 'object') return true;
  try {
    const pkg = JSON.parse(fs.readFileSync(require('path').join(root, 'package.json'), 'utf8'));
    return Boolean(pkg && pkg.name);
  } catch { return false; }
}

function libHasNameOrFileName(libOptions) {
  return Boolean(libOptions && (libOptions.fileName || (typeof libOptions.entry === 'object' && libOptions.entry !== null)));
}

When it happens

Trigger: Running vite build with build.lib configured (entry as a string) but no build.lib.fileName and either no package.json or a package.json missing the name field.

Common situations: Quick library prototypes started outside an npm package directory; monorepo sub-packages whose package.json name was deleted; CI builds that run in a copy of the repo without package.json.

Related errors

• Multiple entry points are not supported when output formats…
• Option "build.lib.name" is required when output formats…
• Environment " " is not defined in the config.
• `input` cannot contain glob characters. They are reserved…
• Name in package.json is required if option…

AI-assisted analysis of vitejs/vite@b4d66fee14 (2026-08-11). Data as JSON: /api/errors/a86e67ee92de62e2. Report an issue: GitHub.

Appendix: source

Thrown at packages/vite/src/node/build.ts:1035

  format: ModuleFormat,
  entryName: string,
  root: string,
  extension?: JsExt,
  packageCache?: PackageCache,
): string {
  if (typeof libOptions.fileName === 'function') {
    return libOptions.fileName(format, entryName)
  }

  const packageJson = findNearestMainPackageData(root, packageCache)?.data
  const name =
    libOptions.fileName ||
    (packageJson && typeof libOptions.entry === 'string'
      ? getPkgName(packageJson.name)
      : entryName)

  if (!name)
    throw new Error(
      'Name in package.json is required if option "build.lib.fileName" is not provided.',
    )

  extension ??= resolveOutputJsExtension(format, packageJson?.type)

  if (format === 'cjs' || format === 'es') {
    return `${name}.${extension}`
  }

  return `${name}.${format}.${extension}`
}

export function resolveBuildOutputs(
  outputs: OutputOptions | OutputOptions[] | undefined,
  libOptions: LibraryOptions | false,
  logger: Logger,
): OutputOptions | OutputOptions[] | undefined {
  if (libOptions) {

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