TypeScript: the typed superset of JS

Notes on TypeScript, written between 2020 and 2022 and revised since. Treat them as working notes rather than a reference: they cover the parts I wanted to pin down, in the order I met them, and the TypeScript Handbook is the authority when you need one.

What they do give you that a handbook page does not: every sample in here is compiled, so if a block says it produces an error, it produces that error.

npm install
npm run check-samples

That extracts every fenced ts block and type-checks it against TypeScript 5.9 under strict. Blocks that are meant to fail, either because they demonstrate a compiler error or because they continue an earlier sample, are listed with a reason in samples.allowlist.txt.

Index

• Intro: what TypeScript is, what it erases, static vs dynamic, and project setup

• Structural Typing: shape-based compatibility, and the excess property check on fresh object literals

• Type Inference and Type Annotation

  • Type Annotation
  • Type Inference
    • Const Assertion

• Types

  • String
  • Number
  • Boolean
  • Union Types (OR)
    • Discriminated Unions
  • Intersection Types (AND)
  • Null
  • Undefined
  • Void
  • Never
  • Any
  • Unknown
  • Object
    • Optional modifier (?)
    • Non-null assertion operator (!)
  • Array
  • Tuple
  • Enum
  • Set

• Type Alias and Interfaces

  • Alias
    • Implements
    • Utility types
      • Readonly
      • Required
      • Record
      • Partial
  • Interfaces
    • Extends
    • Implements

• Classes

  • Classes
    • Access modifier keywords
    • Param properties
  • Inheritance
    • Abstract classes
    • Static members
    • Private Constructors

• Functions

  • Void
    • Void and return extended
  • Function types
  • Function Overloading
  • this parameter
  • Assertion functions

• Type Assertions, Type Casting and Type Declaration

  • Type Assertions
  • Type Casting
  • Type Declarations

• Type Guards

  • Type Guards
    • User defined Type Guards

• Intersection Types

  • Type example with objects
  • Interface example with objects

• Generics

  • Generics
    • Generic constraints