JavaScript vs TypeScript
Javascript vs TypeScript
一句话：TS 是 JS 的超集—— 加了静态类型，编译后就是普通 JS， 运行时没有任何 TS 的东西。
| JavaScript | TypeScript | |
|---|---|---|
| 类型检查 | 运行时才炸 | 编译期就报 |
| 需要构建 | 不需要 | 需要（tsc / esbuild / SWC） |
| IDE 支持 | 靠猜 | 精确补全、跳转、重命名 |
| 重构 | 靠搜字符串 | 改一处，所有不兼容的地方都报出来 |
「运行时没有 TS」这句要强调， 因为它推出两个重要结论：
- 类型不能用来做运行时校验。接口返回的数据是不是真的符合你写的
interface， TS 管不了—— 要校验得用 zod 这类库。这是新手最大的误解。 as断言只是「我保证」， 不做任何检查。滥用as和any等于关掉了 TS。
代价（要主动说）：多一步构建、 有学习成本（泛型、 条件类型、unknown vs any）、 第三方库缺类型时要自己写声明、 复杂类型报错很难读。
会追问：「interface 和type 选哪个？」——interface能被重复声明合并、 更适合描述对象和 class 契约；type 能写联合、 交叉、映射、条件类型，能力更全。 实践上「对象形状用 interface， 其他用 type」，但团队统一比选哪个更重要。
In one line: TS is a superset of JS — it adds static types, and after compilation it is ordinary JS with nothing of TS left at runtime.
| JavaScript | TypeScript | |
|---|---|---|
| Type checking | Fails at runtime | Reported at compile time |
| Build step | Not needed | Needed (tsc / esbuild / SWC) |
| IDE support | Guesswork | Precise completion, go-to-definition, rename |
| Refactoring | Search for strings | Change one place and every incompatible use lights up |
Stress the “nothing of TS at runtime” line, because two important conclusions follow from it:
- Types cannot validate anything at runtime. Whether the data an API returns really matches the
interfaceyou wrote is beyond TS — for that you need something like zod. This is the biggest beginner misconception. - An
asassertion is just “trust me” and checks nothing. Overusingasandanyis the same as turning TS off.
The costs — bring them up yourself: an extra build step, a learning curve (generics, conditional types, unknown vs any), writing your own declarations when a library ships none, and error messages for complex types that are hard to read.
Follow-up: “interface or type?” — interface can be declared again and merged and suits object and class contracts; type can do unions, intersections, mapped and conditional types, so it is more capable. In practice: “interface for object shapes, type for everything else” — but a consistent team choice matters more than which one you pick.