MyCustomError `toString` hacking

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/json2/20121008/json2.min.js"></script>
<h3>Hacking `MyCustomError.prototype.toString()` to polyfill `window.onerror`</h3>

<div id="results"></div>

CSS

#results {
    font-size: 8pt;
}

JavaScript

var a=function(){};
alert({}.toString.call(a));