Count By

Requirements

• Implement countBy(array, iteratee), where iteratee is a callback or a direct property key. Return an ordinary object whose own enumerable properties contain group counts.
• Invoke a callback once per item with (value, index, originalArray). A property-key iteratee reads that direct property, including symbol property keys. Assume a dense array and valid property access.
• Convert every computed key to a string using String, except symbols, which remain symbols. Numeric and string keys with the same spelling share a count. Count every item, including values grouped under false, null, undefined, or an empty string.
• Handle keys such as __proto__, constructor, and toString safely as own count properties without changing the result prototype or any global prototype.
• An empty array returns an empty ordinary object without calling the iteratee. Do not mutate input; preserve the original callback error if it throws. Deep paths and shorthand match predicates are outside scope.

Examples

Input: countBy([6.1, 4.2, 6.3], Math.floor)
Output: { "4": 1, "6": 2 }

Input: countBy([{role:"dev"},{role:"design"},{role:"dev"}], "role")
Output: { dev: 2, design: 1 }