What are type assertions and when should you use them?

Question presented to candidate: "What is a type assertion — value as SomeType — in TypeScript? What does it actually do at run time, when is it legitimate to use one, and what would you reach for instead?"

What a strong answer should cover:

• An assertion tells the compiler to treat an expression as a type; it is erased, so there is no run-time check or conversion ("8080" as unknown as number is still a string)
• Syntax: x as T and <T>x (the angle-bracket form is not allowed in .tsx files)
• The overlap rule: TypeScript rejects assertions between unrelated types (TS2352) and the double assertion as unknown as T bypasses it
• Assertion vs annotation: { id: 1 } as User compiles with a missing property, const u: User = { id: 1 } does not
• Legitimate uses: DOM lookups, Object.keys(obj) as (keyof T)[], data you have already validated, narrowing a library type you know more about
• Safer alternatives: narrowing and type guards, satisfies, annotations, instanceof in catch, validation of untrusted data; related operators ! and as const

Code / implementation expected: Optional — an example showing an assertion that compiles but is wrong at run time, and a safer rewrite.