How do type guards work (typeof, in, instanceof, custom predicates)?

Deep dive

Expanding on the short answer — what usually matters in practice:

• Context (tags): type-guards, narrowing
• 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 "how-do-type-guards-work-(typeof,-in,-instanceof,"
function explain() {
  // Start from the core idea:
  // Type guards narrow a union type at runtime checks. typeof checks primitives, in checks pro
}

Common pitfalls

• Too generic: no concrete trade-offs or examples.
• Mixing average-case and worst-case (e.g., complexity).
• Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

• When would you choose an alternative and why?
• What production issues show up and how do you diagnose them?
• How would you test edge cases?