# L02: Expressions, Values, and Types

Deck: `our_typescript_course_decks/lectures/L02 - Expressions, Values, and Types.pptx`

## Puzzles

1. Trace `baseScore + bonus * 2` when `baseScore = 10` and `bonus = 3`.
2. Write the type of `score`, `"score"`, `score >= 10`, and `String(score)`.
3. Rewrite `level = "3"` as the value and type the program actually needs.

## Puzzle Stations

Use these as quick hands-on checks. Complete a few before writing code.

4. Trace: `3 + 4 * 2`, `(3 + 4) * 2`, `"score" + 8`.
5. Sort: `42`, `"42"`, `true`, `"ready"`, `0`.
6. Boolean: `score >= 10`, `lives === 0`, `name.length > 3`.
7. Domain: choose allowed values for difficulty: any string, or `"easy" | "normal" | "hard"`?
8. Domain: choose allowed values for lives: any number, or whole numbers from 0 to 3?
9. Compiler clue: explain why `const score: number = "10"` fails.
10. Compiler clue: explain why `const ready: boolean = 1` fails.
11. Compiler clue: explain why `const difficulty: "easy" | "normal" | "hard" = "medium"` fails.
12. Repair: fix each failed line without using `any`.
13. Review: reject two AI examples whose type or domain is too broad.


## Compiler Error Challenge

- Copy `labs/snippets/L02-l02-types-starter.ts.txt` to `starter/src/l02-types.ts`.
- Run `npm run build`, read the type errors, then fix the constants.
- Use at least one union type such as `"easy" | "normal" | "hard"`.


## Run If You Touch Code

```sh
cd starter
npm install
npm run build
npm test
```

## Common Bugs

- Guessing a value before tracing the expression.
- Using a broad type when a smaller domain is known.
- Ignoring the compiler message instead of reading the expected and actual types.

## Optional Short Homework

Open [L02 optional homework](../homework/L02.md).

## Bonus Puzzles

- Make a smaller domain with a union type.
- Find a case where string plus number gives the wrong result.
- Ask AI for five typed values and reject the broad or invalid ones.

## Answer Key

1. `baseScore + bonus * 2` with `baseScore = 10` and `bonus = 3` gives `16`, with type `number`.
2. `score` has the declared type of the variable; `"score"` is a string literal; `score >= 10` is `boolean`; `String(score)` is `string`.
3. `level = "3"` stores a string, not a number. If the program needs arithmetic or ordering, use `const level: number = 3`; if only named levels are allowed, use a union such as `type Level = "1" | "2" | "3"`.
4. `3 + 4 * 2` is `11`; `(3 + 4) * 2` is `14`; `"score" + 8` is the string `"score8"`.
5. `42` is a number, `"42"` is a string, `true` is a boolean, `"ready"` is a string, and `0` is a number.
6. `score >= 10`, `lives === 0`, and `name.length > 3` all produce booleans.
7. Difficulty should be a small union such as `"easy" | "normal" | "hard"` if those are the only allowed choices.
8. Lives should be represented as whole numbers from `0` to `3`; TypeScript cannot enforce that exact numeric range by itself without extra modeling or validation.
9. `const score: number = "10"` fails because the value is a string but the variable expects a number.
10. `const ready: boolean = 1` fails because `1` is a number, not a boolean. Use `true` or `false`.
11. `const difficulty: "easy" | "normal" | "hard" = "medium"` fails because `"medium"` is outside the allowed union.
12. One valid repair is `const score: number = 10`, `const ready: boolean = true`, and `const difficulty: Difficulty = "normal"`.
13. Reject AI examples that use types like `string` for known small domains, mix quoted numbers with numbers, or fix errors with `any` instead of choosing the right value or type.