Under normal circumstances, a Web Worker should be passed a URL of the script to execute. However, a local function can also be used

This is done by converting a function to a string, converting that string to a Blob, and converting that Blob to an Object URL.

Code

The code to do this would look a bit like this:

    /* Some code we want to use as Worker */
    const workerCode = () => {
      onmessage = event => {
        postMessage('some message')
      }
    }

    /* Create a Worker from the function */
    const code = workerCode
      .toString()   // Convert the function to a string (Duh)
      .slice(10,-3) // This removes the "() => {" and closing "\n}\n"
    const blob = new Blob([code], {type: 'application/javascript'})
    const worker = new Worker(URL.createObjectURL(blob))

    /* Send a message to the Worker code */
    worker.postMessage('Hello Worker!')

    /* Receive a message from the Worker code */
    worker.onmessage = function(event) {
      event.data // The message received from the worker
    }
    

Be aware that the workerCode does not share the scope with its parent!
As it is a separate script, it also has no window, document, etc.