821. Key By

Implement a keyBy function that creates an object composed of keys generated from the results of running each element through an iteratee.

1. Basic Functionality

• Accept an array and an iteratee (function or property name string)
• Return an object where keys are the iteratee results
• Values are the corresponding array elements
• If multiple elements produce the same key, the last one wins

2. Iteratee Types

• If iteratee is a function, call it with each element
• If iteratee is a string, use it as a property key

Examples

keyBy([{ id: 'a', name: 'Alice' }, { id: 'b', name: 'Bob' }], 'id');
// Returns: { a: { id: 'a', name: 'Alice' }, b: { id: 'b', name: 'Bob' } }

keyBy([1.2, 2.4, 3.6], Math.floor);
// Returns: { 1: 1.2, 2: 2.4, 3: 3.6 }

Use Case

This is commonly used to normalize API responses for efficient lookup by ID, especially in Redux/state management.