TypeScript Type System
TypeScript's static type system: types, interfaces, generics, utility types, and advanced type manipulation.
The TypeScript type system's main concepts, from primitive types, interfaces and generics to union and intersection types, type guards, utility types, discriminated unions, mapped and conditional types and template literal types, each with a description and example.
Use it when
- You need the TypeScript construct for a typing problem and an example.
- You are reading advanced type definitions in a library.
Watch out for
- TypeScript's types are erased at runtime; they do not validate data coming from outside.
- Complex conditional and mapped types can slow the compiler and are hard to read; use them with care.
The data
Concepts
12 rows
| Name | Types | Description | Example | Keyword | Patterns | Common | Pattern |
|---|---|---|---|---|---|---|---|
| Primitive Types |
| Basic built-in types. TypeScript adds `any`, `unknown`, `never`, `void`. | let name: string = "Alice"; let age: number = 30; let isActive: boolean = true; | null | null | null | null |
| Interfaces | null | Define object shape; can be extended and implemented. Structural typing. | interface User { id: number; name: string; email?: string; } const u: User = { id: 1, name: "Bob" }; | interface | null | null | null |
| Type Aliases | null | Create new named type; can represent primitives, unions, tuples, objects. | type ID = number | string; type Point = [number, number]; type User = { name: string }; | type | null | null | null |
| Generics | null | Type variables enable reusable components that work with multiple types. | function identity<T>(arg: T): T { return arg; } let output = identity<string>("hello"); | <T> | null | null | null |
| Union Types | null | Value can be one of several types; use type guards to narrow. | function format(x: string | number) { if (typeof x === 'string') { ... } else { ... } } | | | null | null | null |
| Intersection Types | null | Combine multiple types into one; object must satisfy all constraints. | type Admin = User & { permissions: string[] }; | & | null | null | null |
| Type Guards | null | Narrow types within conditional blocks. | function isString(x: unknown): x is string { return typeof x === 'string'; } if (isString(val)) { val.toUpperCase(); } | null |
| null | null |
| Utility Types | null | Built-in type transformations; avoid manual re-declaration. | interface Post { title: string; content: string; published: boolean; } type EditPost = Pick<Post, 'title' | 'content'>; | null | null |
| null |
| Discriminated Unions | null | Union of types with common literal field; enables exhaustive type checking in switch. | type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; side: number }; function area(s: Shape) { switch(s.kind) { case 'circle': return Math.PI * s.radius ** 2; ... } } | null | null | null | tagged union |
| Mapped Types | null | Create types by mapping over properties of another type. | type Readonly<T> = { readonly [P in keyof T]: T[P]; } type Optional<T> = { [P in keyof T]?: T[P] }; | keyof, in | null | null | null |
| Conditional Types | null | Types that choose between two types based on a condition. | type IsString<T> = T extends string ? true : false; type NonNullable<T> = T extends null | undefined ? never : T; | T extends U ? X : Y | null | null | null |
| Template Literal Types | null | Build strings from other types; useful for CSS property names, API endpoints. | type EventName = `on${Capitalize<string>}`; // onLoad, onMouseEnter, etc. | `${}` | null | null | null |
Fields
The shape of this dataset, read from the data itself: every column, its type and a real value. Agents can use this to plan a query without fetching the whole document.
| Section | Field | Type | Example |
|---|---|---|---|
| concepts | name | string | Primitive Types |
| concepts | types | array | [7 items] |
| concepts | description | string | Basic built-in types. TypeScript adds `any`, `u… |
| concepts | example | string | let name: string = "Alice"; let age: number = 3… |
| concepts | keyword | string | interface |
| concepts | patterns | array | [4 items] |
| concepts | common | array | [6 items] |
| concepts | pattern | string | tagged union |
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