Function mergeConfigRecursively

packages/vite/src/node/utils.ts:1404–1507

(
  defaults: Record<string, any>,
  overrides: Record<string, any>,
  rootPath: string,
)

}

function mergeConfigRecursively(
  defaults: Record<string, any>,
  overrides: Record<string, any>,
  rootPath: string,
) {
  const merged: Record<string, any> = { ...defaults }
  if (rollupOptionsRootPaths.has(rootPath)) {
    setupRollupOptionCompat(merged, rootPath)
  }
  if (rootPath === 'server') {
    setupHmrWsOptionCompat(merged)
  }
  if (rootPath === 'server.hmr') {
    for (const key of wsOptionKeys) {
      Object.defineProperty(
        merged,
        key,
        Object.getOwnPropertyDescriptor(defaults, key)!,
      )
    }
  }

  for (const key in overrides) {
    const value = overrides[key]
    if (value == null) {
      continue
    }

    let existing = merged[key]
    if (key === 'rollupOptions' && rollupOptionsRootPaths.has(rootPath)) {
      // if both rollupOptions and rolldownOptions are present,
      // ignore rollupOptions and use rolldownOptions
      if (overrides.rolldownOptions) continue
      existing = merged.rolldownOptions
    }

    if (existing == null) {
      if (rootPath === '' && key === 'environments' && isObject(value)) {
        // Clone to avoid mutating the original override object
        const environments = { ...value }
        for (const envName in environments) {
          environments[envName] = setupRollupOptionCompatForEnvironment(
            environments[envName],
          )
        }
        merged[key] = environments
      } else if (rootPath === 'environments') {
        // `environments` exists, but a new environment is added
        merged[key] = setupRollupOptionCompatForEnvironment(value)
      } else {
        merged[key] = value
      }
      continue
    }

    // fields that require special handling
    if (key === 'alias' && (rootPath === 'resolve' || rootPath === '')) {
      merged[key] = mergeAlias(existing, value)