Array Grouping (original) (raw)

callbackfn should be a function that accepts two arguments. groupBy calls callbackfn once for each element in items, in ascending order, and constructs a new Object of arrays. Each value returned by callbackfn is coerced to a property key, and the associated element is included in the array in the constructed object according to this property key.

callbackfn is called with two arguments: the value of the element and the index of the element.

The return value of groupBy is an object that does not inherit from %Object.prototype%.