我们可以使用Web Workers来在后台运行async函数。下面是一个示例代码，其中我们创建了一个新的Web Worker并传递了我们要运行的async函数。

// async function to be run in background
async function compute() {
  // some heavy computations here...
  return result;
}

// creating a new Web Worker
const myWorker = new Worker('worker.js');

// sending the async function to the worker
myWorker.postMessage({ type: 'compute', data: compute.toString() });

// receiving the result from the worker
myWorker.onmessage = (event) => {
  const result = event.data;
  // do something with the result...
}

在worker.js文件中，我们处理传递过来的async函数并调用它，然后将结果发送回主线程。

// handling messages from the main thread
onmessage = async (event) => {
  const { type, data } = event.data;
  switch (type) {
    case 'compute':
      // converting the async function to a regular function
      const func = new Function(`return (${data})()`);
      const result = await func();
      postMessage(result);
      break;
  }
};

这样我们就可以在JavaScript中在后台运行async函数了。