Injected JS assigning window.onerror fails to fire

Question

I need to run third party JS to scan jserrors. I am running the following in chrome's console

window.onerror = function(errorMsg, url, lineNumber) {
  alert(errorMsg);
}

throw new Error('foo');

I expect both the console to inform me of the error and a alert to pop up displaying the error message. The alert is never fired, and I assume the event is not being fired. why not?

Answer 1

The Chrome Developer Console won't call the user-defined error event listener if the error is thrown immediately (according to this answer by Paul S.).

Throwing the error "just in time":

> window.onerror = function () {console.log('error!');};
function () {console.log('error!');}
> throw new Error();
Error

Throwing the error deferred:

> window.setTimeout(function() {throw new Error()}, 0);
xxxx
error!
Uncaught Error

(xxxx is the specific return value of setTimeout())

---

Here is the original code snippet which led me to the point that the Chrome Dev Console changes the internal behaviour somehow:

// #btn is a simple button
document.getElementById("btn").addEventListener("click", function () {
    var script = document.createElement("script");
    script.innerHTML = "window.onerror=function(){alert('An error occurred!');};throw new Error('42');";
    document.head.appendChild(script);
});

This code is to be executed in a normal HTML file.