09 — Type Assertions & Narrowing

Core Concepts

• as keyword — tells TS to treat a value as a specific type
• Non-null assertion (!) — tells TS a value is not null/undefined
• typeof guards — typeof x === "string" narrows primitives
• instanceof guards — x instanceof Class narrows to class instances
• in operator — "key" in obj narrows to types that have that property
• Custom type guards — function isX(val): val is X user-defined narrowing
• Assertion functions — asserts val is X throws if not true, narrows after
• Discriminated unions — shared literal field for switch narrowing
• Control flow analysis — TS tracks type through if/else/return/throw

Type Safety Hierarchy