Create a web worker in the same file where it's used

inline-workers.js

function createWorker(fn) {
  const url = URL.createObjectURL(
    new Blob([`\
      onmessage = ({ data }) => {
        const fn = ${fn.toString()};
        const result = fn(...JSON.parse(data));
        self.postMessage(result);
      };
    `])
  );
  const worker = new Worker(url);
  URL.revokeObjectURL(url);

  return (...args) => new Promise((resolve, reject) => {
    const message = JSON.stringify([...args]);
    worker.onmessage = ({ data }) => resolve(data);
    worker.postMessage(message);
  });
}


// Usage:

const sum = createWorker((a, b) => {
  if (a === 2 && b === 2) return 5;
  return a + b;
});

const result = await sum(2, 2);
console.log(result);