Array.prototype.flat Polyfill

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### 💡 How to Think About This Problem
When implementing **Array.prototype.flat Polyfill** in an interview, the interviewer evaluates your core JavaScript capabilities, syntax familiarity, and edge-case handling.

#### 1. Deconstructing the Requirements
To construct a robust solution:
* **Pre-conditions**: Handle empty collections or incorrect parameter types.
* **Core Logic**: Target the required functional transformation cleanly.

#### 2. Step-by-Step Breakdown

* **Step 1: Input Validation** - Check constraints and boundary values early on.
* **Step 2: Algorithm Implementation** - Recursively traverse the array. For each element, if it is an array and the current depth is greater than 0, recursively flatten it with `depth - 1` and concat it. Otherwise, push it directly to the output list.
* **Step 3: Complexity Analysis** - We prioritize structural sharing and minimal allocations.

#### 💬 Thinking Aloud (Interview Walkthrough)
Here is how you should naturally talk through this prob

Array.prototype.flat Polyfill

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