rollup-plugin-netlify-modulepreload

plugin.js

import * as path from "path";
import * as fs from "fs";

export default function netlifyModulePreload(config) {
  return {
    name: "netlify-modulepreload",
    generateBundle(outputOptions, bundle) {
      const { publicRoot, entries } = config;

      const { dir, format } = outputOptions;
      if (format !== "es") return;

      const rootPath = path.resolve("/", path.relative(publicRoot, dir));

      const paths = Object.keys(entries);
      const pathsObj = paths.reduce((acc, cur) => {
        const pathsObj = entries[cur].reduce(
          (a, c) => ({ ...a, [c]: new Set() }),
          {}
        );
        return { ...acc, ...pathsObj };
      }, {});

      Object.keys(bundle)
        .filter(bn => paths.some(p => bn.includes(p)))
        .forEach(target => {
          const files = [target, ...bundle[target].imports];

          const name = target.split(".")[0];
          entries[name].forEach(path => {
            files.forEach(file => {
              pathsObj[path].add(file);
            });
          });
        });

      const entryFiles = pathsObj["*"];
      Object.keys(pathsObj).forEach(path => {
        if (path === "*") return;
        entryFiles.forEach(file => {
          pathsObj[path].add(file);
        });
      });

      delete pathsObj["*"];

      const outputLines = [];
      Object.keys(pathsObj).forEach(url => {
        outputLines.push(url);

        const files = [...pathsObj[url]];
        files.forEach(file => {
          const resolvedFile = path.resolve(rootPath, file);
          outputLines.push(`\tLink: <${resolvedFile}>; rel=modulepreload`);
        });

        outputLines.push("");
      });

      fs.writeFileSync(
        path.resolve(__dirname, publicRoot, "_headers"),
        outputLines.join("\n"),
        { encoding: "utf8" }
      );
    }
  };
}