Why can't Web Worker call a function directly?

Question

We can use the web worker in HTML5 like this:

var worker = new Worker(\'worker.js\');

but why can\'t we call a function like this?

Answer 1

This question has been asked before, but for some reason, the OP decided to delete it.
I repost my answer, in case one needs a method to create a Web worker from a function.

---

In this post, three ways were shown to create a Web worker from an arbitrary string. In this answer, I'm using the third method, since it's supported in all environments.

A helper file is needed:

// Worker-helper.js
self.onmessage = function(e) {
    self.onmessage = null; // Clean-up
    eval(e.data);
};

In your actual Worker, this helper file is used as follows:

// Create a Web Worker from a function, which fully runs in the scope of a new
//    Worker
function spawnWorker(func) {
    // Stringify the code. Example:  (function(){/*logic*/}).call(self);
    var code = '(' + func + ').call(self);';
    var worker = new Worker('Worker-helper.js');
    // Initialise worker
    worker.postMessage(code);
    return worker;
}

var worker = spawnWorker(function() {
    // This function runs in the context of a separate Worker
    self.onmessage = function(e) {
        // Example: Throw any messages back
        self.postMessage(e.data);
    };
    // etc..
});
worker.onmessage = function() {
    // logic ...
};
worker.postMessage('Example');

Note that the scopes are strictly separated. Variables can only be passed and forth using worker.postMessage and worker.onmessage. All messages are structured clones.