TypeScript error TS2532 'Object is possibly undefined' (also 'Object is possibly null', TS18048 "'x' is possibly 'undefined'") after enabling strict. What is the right fix?

Cause: With strictNullChecks on (included in strict), null and undefined are separate types. If a value's type includes them, for example the result of array.find(), an optional property (name?: string), a Map.get() or document.querySelector(), TypeScript refuses to let you use it as if it were definitely present, because at runtime it may not be. The compiler is pointing at a real possible crash. Fix (best first): 1. Narrow with a check: if (user) { use(user.age) }, or return early: if (!user) return. For null and undefined only, use user != null or user !== undefined (careful: a plain truthiness test also rejects 0 and an empty string). 2. Use optional chaining and a default: const age = user?.age ?? 0. 3. Model the data better so the state is impossible, for example a discriminated union (a kind field) instead of optional properties that are sometimes required. 4. Throw a clear error where absence is a bug: if (!el) throw new Error('missing #app element'), which also narrows the type afterwards. 5. The non-null assertion (user!.age) silences the compiler without checking anything, so a wrong assumption becomes a runtime TypeError. Use it only where you truly know the value exists, and prefer the options above. 6. Do not turn off strictNullChecks or strict just to clear the errors; you lose the protection for the entire project. Verify: run tsc --noEmit (or your editor's check). The error disappears and each possibly-missing value has an explicit check or default. Sources: https://www.typescriptlang.org/tsconfig/#strictNullChecks https://www.typescriptlang.org/docs/handbook/2/narrowing.html