Act as a senior JavaScript developer with 10+ years of experience. Explain the key differences between `let`, `var`, and `const` in JavaScript to a [beginner/intermediate/advanced] programmer. Cover topics like [hoisting], [scope (block vs. function)], and [mutability/reassignment]. Provide [real-world examples] or [common pitfalls] to illustrate your points. Ensure your explanation is clear, concise, and tailored to the learner's level. Highlight when to use each declaration and why modern best practices favor `let` and `const` over `var`.