Function renderChunk

packages/plugin-legacy/src/index.ts:482–633

(raw, chunk, opts, { chunks })

    },

    async renderChunk(raw, chunk, opts, { chunks }) {
      if (config.build.ssr) {
        return null
      }

      // On first run, initialize the map with sorted chunk file names
      let chunkFileNameToPolyfills = outputToChunkFileNameToPolyfills.get(opts)
      if (chunkFileNameToPolyfills == null) {
        chunkFileNameToPolyfills = new Map()
        for (const fileName in chunks) {
          chunkFileNameToPolyfills.set(fileName, {
            modern: new Set(),
            legacy: new Set(),
          })
        }
        outputToChunkFileNameToPolyfills.set(opts, chunkFileNameToPolyfills)
      }
      const polyfillsDiscovered = chunkFileNameToPolyfills.get(chunk.fileName)
      if (polyfillsDiscovered == null) {
        throw new Error(
          `Internal @vitejs/plugin-legacy error: discovered polyfills for ${chunk.fileName} should exist`,
        )
      }

      if (!isLegacyChunk(chunk)) {
        if (
          options.modernPolyfills &&
          !Array.isArray(options.modernPolyfills) &&
          genModern
        ) {
          // analyze and record modern polyfills
          await detectPolyfills(
            raw,
            modernTargets,
            assumptions,
            polyfillsDiscovered.modern,
          )
        }

        const ms = new MagicString(raw)

        if (genLegacy && chunk.isEntry) {
          // append this code to avoid modern chunks running on legacy targeted browsers
          ms.prepend(createModernChunkLegacyGuard(chunk.fileName))
        }

        if (raw.includes(legacyEnvVarMarker)) {
          const re = new RegExp(legacyEnvVarMarker, 'g')
          let match
          while ((match = re.exec(raw))) {
            ms.overwrite(
              match.index,
              match.index + legacyEnvVarMarker.length,
              `false`,
            )
          }
        }