Use array.[] instead of array.@each

In Ember, I knew array.@each, array.length, and array.[] would all work in my computed properties, but it turns out array.[] and array.length are much more performant, and should be used over array.@each in almost every case.

Use array.@each when you need to know about a property/properties on a member of the array changing, for example: array.@each.{foo,bar,baz} will watch for changes to the foo, bar, and baz properties.

https://blog.eladshahar.com/performant-array-computed-properties.html

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