# Strict Mode & Compiler Errors — TypeScript Fundamentals

Source: https://www.skillbyai.com/en/typescript/ts-strict-mode

> Turn on strict mode, handle null and undefined explicitly, and read TypeScript compiler errors to find the fix fast.

## The strict flag

`"strict": true` in `tsconfig.json` bundles several checks — `strictNullChecks`, `noImplicitAny`, and more — into one switch. New projects should turn it on from day one.

## strictNullChecks catch

With strict null checks, `null`/`undefined` aren't silently allowed everywhere — you must handle them explicitly.

```typescript
function getLength(s: string | null): number {
  return s.length;
  // Error: 's' is possibly 'null'.
}

function getLengthSafe(s: string | null): number {
  return s ? s.length : 0; // narrowed, OK
}
```

## Reading compiler errors

TS error messages name the exact incompatible types ("Type 'X' is not assignable to type 'Y'") — read the two type names first; they usually point straight at the fix.

**Quiz:** What does `"strict": true` in tsconfig.json do?

- [ ] Enables a single extra check
- [x] Bundles several strict type-checking options together
- [ ] Only affects the output file names
- [ ] Disables type checking in test files

*Answer:* Bundles several strict type-checking options together. strict is a shorthand that turns on strictNullChecks, noImplicitAny, and several other checks at once.

## Final quiz 1 of 8

Final quiz

**Quiz:** Which `tsconfig.json` option enables the full set of strict checks?

- [ ] noEmit
- [x] strict
- [ ] target
- [ ] outDir

*Answer:* strict. `strict: true` bundles strictNullChecks, noImplicitAny and more.

## Final quiz 2 of 8

Final quiz

**Quiz:** Which type should you prefer over `any` for unpredictable data?

- [x] unknown
- [ ] object
- [ ] Function
- [ ] never

*Answer:* unknown. `unknown` forces you to narrow before use.

## Final quiz 3 of 8

Final quiz

**Quiz:** What does a tuple like `[string, number]` enforce?

- [ ] Any length of mixed values
- [x] A fixed length with a type at each position
- [ ] Only string values
- [ ] Readonly access

*Answer:* A fixed length with a type at each position. Tuples fix both length and per-position types.

## Final quiz 4 of 8

Final quiz

**Quiz:** How do you let TypeScript narrow a union by a shared field?

- [x] Use a discriminated union with a literal `kind`
- [ ] Cast with `as any`
- [ ] Use `Partial<T>`
- [ ] Declare the field optional

*Answer:* Use a discriminated union with a literal `kind`. A literal discriminant lets `switch` narrow each variant.

## Final quiz 5 of 8

Final quiz

**Quiz:** What does `Partial<User>` produce?

- [x] A type with every property optional
- [ ] A type with every property readonly
- [ ] A type with only required keys
- [ ] A union of keys

*Answer:* A type with every property optional. `Partial<T>` makes all properties optional.

## Final quiz 6 of 8

Final quiz

**Quiz:** What does `T extends { length: number }` do?

- [x] Constrains T to types that have a length property
- [ ] Makes T a number
- [ ] Adds a length field at runtime
- [ ] Disables inference

*Answer:* Constrains T to types that have a length property. Constraints narrow what a generic accepts while staying reusable.

## Final quiz 7 of 8

Final quiz

**Quiz:** What does `import type { Point }` do to the output JavaScript?

- [ ] Adds a runtime import
- [x] Is erased at compile time
- [ ] Bundles the type
- [ ] Creates a class

*Answer:* Is erased at compile time. Type-only imports are removed from the emitted JS.

## Final quiz 8 of 8

Final quiz

**Quiz:** What type does `await` give you for `Promise<User>`?

- [ ] Promise<User>
- [x] User
- [ ] unknown
- [ ] void

*Answer:* User. `await` unwraps the Promise to its resolved type.