Group By

Requirements

• Implement groupBy(array, iteratee). iteratee is a callback or a direct property key. Return an ordinary object containing own enumerable arrays of grouped values. Assume dense input arrays.
• Call a callback once per item with (value, index, originalArray). A property key reads that direct property, including symbols. Deep paths and matching shorthands are outside scope.
• Computed symbol keys remain symbols. Convert every other key with String, so numeric and string keys with the same spelling share a group. Include falsy and missing keys.
• Preserve input order within each group and original item identities. Return new group arrays, and do not mutate input. Empty input returns a fresh empty ordinary object without invoking the callback.
• Safely support keys such as __proto__, constructor, and toString without changing any prototype. Propagate callback errors unchanged.

Examples

Input: groupBy([6.1,4.2,6.3], Math.floor)
Output: { "4": [4.2], "6": [6.1,6.3] }

Input: groupBy([{team:"a",id:1},{team:"b",id:2},{team:"a",id:3}], "team")
Output: { a: [{team:"a",id:1},{team:"a",id:3}], b: [{team:"b",id:2}] }