this code from amazon.com's source is... well, just... what web development is all about

awesome.js

document.write = (function(write){
  var override = function() {
    if(document.readyState !== "loading") { // document has finished loading - we want to intercept this call to document.write
      if (window.ueLogError) {
        try {
          throw new Error("`document.write` called after page load on the gateway.");
        }
        catch(e) {
          ueLogError(e, { logLevel : 'ERROR', attribution: 'gw-third-party-js' });
        }
      }
    }
    else if(write.apply) { // modern browsers
      write.apply(document, Array.prototype.slice.call(arguments, 0));
    }
    else { // old versions of IE
      document.write = write;
      var args = "";
      for(var a=0; a<arguments.length; a++) {
        args += (args?",":"")+"arguments["+a+"]";
      }
      eval("document.write("+args+");");
      document.write = override;
    }
  };
  return override;
}(document.write));

(I didn't write this code)

Want to prevent from being overwritten by third-party javascript?

This is their little dirty secret.

Could you explain the purpose of this code ? What is really about ?

@rbecheras if doc.write is called after the page has loaded, the document will be overwritten :/ . This tries to avoid that scenario, e.g. from third party js sources. Check out https://developer.mozilla.org/en-US/docs/Web/API/document.write

Oh lol, one of my clients had this code in their page...