What is type narrowing and what are type guards?

Question presented to candidate: "TypeScript lets you write if (typeof x === 'string') and then call string methods on x inside the block. What is that mechanism called, which checks does the compiler understand, and how do you write your own type guard?"

What a strong answer should cover:

• Narrowing is control-flow analysis: a variable has a declared type and, at each point in the code, a narrower current type based on the checks and assignments that came before
• The built-in guards: typeof, truthiness, === / !== (including null checks), instanceof, Array.isArray, in, and discriminant property checks; early returns narrow the rest of the function
• Classic traps: typeof null === "object", truthiness also drops "" and 0, narrowing of mutable values is not trusted inside callbacks
• User-defined type guards (x is T): the false branch narrows too, filter(isX) gives a narrowed array, and the compiler trusts the predicate blindly, so a wrong guard is a real bug
• Related tools and versions: assertion functions (asserts x is T), 5.4 keeping narrowing in closures after the last assignment, 5.5 inferring predicates

Code / implementation expected: Yes — a function that narrows a union with built-in checks, plus one hand-written x is T guard.