JS Built-ins
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ArrayArray.prototype.keysadvanced

Array.prototype.keys() — build an index map

📖 Usage Example
const arr = ['a', 'b']
[...arr.keys()]     // → [0, 1]           (indices only)
[...arr.entries()]  // → [[0,'a'],[1,'b']] (index+value pairs)

Use keys() for indices only; use entries() when you need both the index and the value.

Array.prototype.keys() — index map

keys() gives you just the indices, while entries() gives you [index, value] pairs. When you only need the indices to look up values manually, keys() is the right choice.

Challenge: Implement indexMap(arr) that returns an object mapping each numeric index to its corresponding array value.

indexMap(['a', 'b', 'c']) // → { 0: 'a', 1: 'b', 2: 'c' }
indexMap([])              // → {}
💡 Hints (3)
  • • Use `for (const i of arr.keys())` to get each index.
  • • Assign `result[i] = arr[i]` inside the loop to populate the map.
  • • `keys()` only yields indices — you still need `arr[i]` to get the value.