The goal of TypeScript is to be a static typechecker for JavaScript programs - in other words, a tool that runs before your code runs (static) and ensure that the types of the program are correct (typechecked).
TypeScript offers all of JavaScript’s features, and an additional layer on top of these: TypeScript’s type system. This means that your existing working JavaScript code is also TypeScript code. The main benefit of TypeScript is that it can highlight unexpected behavior in your code, lowering the chance of bugs.
primitive types，除了javascript的八种类型，typescript 还提供：
- any
- unknown
- never
- void
- enumeration
There are two syntaxes for building types: Interfaces and Types. You should prefer interface. Use type when you need specific features.
With TypeScript, you can create complex types by combining simple ones. There are two popular ways to do so: with Unions, and with Generics.
Structural Type System
One of TypeScript’s core principles is that type checking focuses on the shape that values have. This is sometimes called “duck typing” or “structural typing”.
In a structural type system, if two objects have the same shape, they are considered to be of the same type.
const a = 0console.log("hi")
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