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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.
Sencha - Senior Forum Manager
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?"