Function getLegacyOutputFileName

packages/plugin-legacy/src/index.ts:416–448

(
        fileNames:
          | string
          | ((chunkInfo: Rollup.PreRenderedChunk) => string)
          | undefined,
        defaultFileName = '[name]-legacy-[hash].js',
      )

      }

      const getLegacyOutputFileName = (
        fileNames:
          | string
          | ((chunkInfo: Rollup.PreRenderedChunk) => string)
          | undefined,
        defaultFileName = '[name]-legacy-[hash].js',
      ): string | ((chunkInfo: Rollup.PreRenderedChunk) => string) => {
        if (!fileNames) {
          return path.posix.join(config.build.assetsDir, defaultFileName)
        }

        return (chunkInfo) => {
          let fileName =
            typeof fileNames === 'function' ? fileNames(chunkInfo) : fileNames

          if (fileName.includes('[name]')) {
            // [name]-[hash].[format] -> [name]-legacy-[hash].[format]
            fileName = fileName.replace('[name]', '[name]-legacy')
          } else if (nonLeadingHashInFileNameRE.test(fileName)) {
            // custom[hash].[format] -> [name]-legacy[hash].[format]
            // custom-[hash].[format] -> [name]-legacy-[hash].[format]
            // custom.[hash].[format] -> [name]-legacy.[hash].[format]
            // custom.[hash:10].[format] -> custom-legacy.[hash:10].[format]
            fileName = fileName.replace(prefixedHashInFileNameRE, '-legacy$&')
          } else {
            // entry.js -> entry-legacy.js
            // entry.min.js -> entry-legacy.min.js
            fileName = fileName.replace(/(.+?)\.(.+)/, '$1-legacy.$2')
          }

          return fileName
        }
      }

      const createLegacyOutput = (
        options: Rollup.OutputOptions = {},