Question

I am currently writing a little library in JavaScript to help me delegate to a web-worker some heavy computation .

For some reasons (mainly for the ability to debug in the UI thread and then run the same code in a worker) I’d like to detect if the script is currently running in a worker or in the UI thread.

I’m not a seasoned JavaScript developper and I would like to ensure that the following function will reliably detect if I’m in a worker or not :

function testenv() {
    try{
        if (importScripts) {
            postMessage("I think I'm in a worker actually.");
        }
    } catch (e) {
        if (e instanceof ReferenceError) {
            console.log("I'm the UI thread.");
        } else {
            throw e;
        }
    }
}

So, does it ?

Answer 1

As noted there is an answer in another thread which says to check for the presence of a document object on the window. I wanted to however make a modification to your code to avoid doing a try/catch block which slows execution of JS in Chrome and likely in other browsers as well.

EDIT: I made an error previously in assuming there was a window object in the global scope. I usually add

//This is likely SharedWorkerContext or DedicatedWorkerContext
window=this;

to the top of my worker loader script this allows all functions that use window feature detection to not blow up. Then you may use the function below.

function testEnv() {
  if (window.document === undefined) {
    postMessage("I'm fairly confident I'm a webworker");
  } else {
    console.log("I'm fairly confident I'm in the renderer thread");
  }
}

Alternatively without the window assignment as long as its at top level scope.

var self = this;
function() {
  if(self.document === undefined) {
    postMessage("I'm fairly confident I'm a webworker");
  } else {
    console.log("I'm fairly confident I'm in the renderer thread");
  }
}