You found a bug! We've classified it as
We encourage you to continue the discussion and to find an acceptable workaround while we work on a permanent fix.
The Ext.Array.sum polyfill method should be very similar to the Array.reduce method.
I suggested adding the callback and initialValue params and delegating to Array.reduce when possible.
Another option is to leave Ext.Array.sum as is and add the new Ext.Array.reduce method.
Pushed as a story. Thank you!
I was searching what Google did... and if they implemented Array.reduce...
I found this funny inline comment:
"We're google; we can't have "map" without "reduce" can we?"