undefined means a variable was declared but not assigned; null is an explicit empty value. typeof null is a historical "object".
Advanced answer
Deep dive
Expanding on the short answer — what usually matters in practice:
Context (tags): types, null, undefined
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.
Examples
A tiny example (an explanation template):
// Example: discuss trade-offs for "explain-null-vs-undefined-in-javascript."
function explain() {
// Start from the core idea:
// undefined means a variable was declared but not assigned; null is an explicit empty value.
}
Common pitfalls
Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).