# TypeScript — JavaScript with Syntax for Types

> TypeScript is a superset of JavaScript that adds static types, interfaces, generics, and modern ES features, compiled down to plain JavaScript. Created by Anders Hejlsberg at Microsoft. Used by Airbnb, Slack, Asana, and a huge share of modern web projects.

## Install

Save as a script file and run:

## Quick Use

```bash
# Install globally
npm i -g typescript

# Or in a project
npm i -D typescript
npx tsc --init
```

Example:
```ts
// user.ts
interface User {
  id: number;
  email: string;
  role: "admin" | "user";
}

function greet(user: User): string {
  return `Hello, ${user.email} (${user.role})`;
}

const me: User = { id: 1, email: "w@tokrepo.com", role: "admin" };
console.log(greet(me));
```

Compile:
```bash
npx tsc                      # Compile once
npx tsc --watch              # Watch mode
npx tsc --noEmit             # Type-check only
```

`tsconfig.json` essentials:
```json
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "declaration": true,
    "outDir": "dist"
  },
  "include": ["src/**/*"]
}
```

## Intro

TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. Designed by Anders Hejlsberg (also creator of C# and Turbo Pascal) at Microsoft. First released in 2012, now used in most large-scale web projects. TypeScript makes JavaScript scalable for teams and big codebases by catching type errors at compile time and providing world-class IDE autocomplete and refactoring.

- **Repo**: https://github.com/microsoft/TypeScript
- **Stars**: 108K+
- **License**: Apache 2.0

## What TypeScript Does

- **Static types** — interfaces, type aliases, unions, intersections
- **Generics** — parameterized types for reusable code
- **Type inference** — infer types from values and signatures
- **Type narrowing** — refine types via checks (typeof, instanceof, discriminators)
- **Utility types** — Partial, Required, Pick, Omit, Record, Awaited
- **Declaration files** — `.d.ts` for typing existing JS libraries
- **Module resolution** — classic, node, bundler strategies
- **JSX support** — TSX for React, Vue, Solid
- **Project references** — incremental builds across packages
- **Strict mode** — strict null checks, no implicit any

## Architecture

TSC compiler written in TypeScript itself. Parses source to AST, performs type checking using a structural type system, then emits JavaScript. Language Server (tsserver) powers IDE features via LSP.

## Self-Hosting

Language tool, ships via npm.

## Key Features

- Structural type system
- Type inference
- Generics
- Discriminated unions
- Utility types
- Declaration files for JS libraries
- JSX/TSX support
- Project references
- Incremental builds
- Rich LSP integration

## Comparison

| Language | Types | Compiles To | Runtime |
|---|---|---|---|
| TypeScript | Static + inference | JavaScript | Node/Browser/Bun/Deno |
| Flow | Static | JavaScript | Same |
| Hegel | Static | JavaScript | Same |
| ReScript | Static | JavaScript | Same |
| PureScript | Static | JavaScript | Same |
| Elm | Static | JavaScript | Browser |

## 常见问题 FAQ

**Q: 什么时候用 TypeScript？**
A: 任何 JS 项目超过 1000 行、团队规模 > 1、长期维护。小脚本可以不用。

**Q: Strict 模式必须开吗？**
A: 强烈建议。关掉严格模式等于放弃 TypeScript 的核心价值。可以用 `@ts-expect-error` 标注临时例外。

**Q: 编译慢？**
A: v5+ 引入 project references 和增量构建。大项目用 `tsc -b` 构建模式。也可用 swc、esbuild 只做转换跳过类型检查（配合 `tsc --noEmit` 单独检查）。

## 来源与致谢 Sources

- Docs: https://www.typescriptlang.org/docs
- GitHub: https://github.com/microsoft/TypeScript
- License: Apache 2.0

---
Source: https://tokrepo.com/en/workflows/1b748b68-35fe-11f1-9bc6-00163e2b0d79
Author: Script Depot