How to detect on-page 404 errors using JavaScript?

Question

I have an HTML page where several JavaScript, CSS and images files are referenced. These references are dynamically injected and user can manually copy the HTML page and the

Answer 1

@alexander-omara gave the solution.

You can even add it in many files but the window handler can/should be added once.

I use the singleton pattern to achieve this:

some_global_object = {
  error: (function(){
     var activate = false;
     return function(enable){
        if(!activate){
           activate = true;
           window.addEventListener('error', function(e){
              // maybe extra code here...
              // if(e.target.custom_property)
              // ...
           }, true);
        }
        return activate;
     };
  }());

Now, from any context call it as many times you want as the handler is attached only once:

some_global_object.error();