TypeScript — JavaScript with Syntax for Types

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

FAQ

Q: When should I use TypeScript? A: Any JS project beyond 1000 lines, team size > 1, or long-term maintenance. Small scripts can skip it.

Q: Must I enable strict mode? A: Strongly recommended. Turning off strict mode essentially discards TypeScript's core value. Use @ts-expect-error to annotate temporary exceptions.

Q: Slow compilation? A: v5+ introduces project references and incremental builds. For large projects use tsc -b build mode. You can also use swc or esbuild for transformation only and skip type checking (combined with a separate tsc --noEmit check).

Sources

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