TypeError: window.initMap is not a function

Actually the error is being generated by the initMap in the Google’s api script

 <script async defer
  src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap">
</script>

so basically when the Google Map API is loaded then the initMap function is executed.
If you don’t have an initMap function, then the initMap is not a function error is generated.

So basically what you have to do is one of the following options:

1. to create an initMap function
2. replace the callback function with one of your own that created for the same purpose but named it something else
3. replace the &callback=angular.noop if you just want an empty function() (only if you use angular)