TypeScript Quick Reference

// Numeric enum (auto-increments from 0)
enum Direction {
  Up,       // 0
  Down,     // 1
  Left,     // 2
  Right     // 3
}

// Numeric enum with custom start
enum HttpStatus {
  OK = 200,
  Created = 201,
  BadRequest = 400,
  NotFound = 404,
  ServerError = 500
}

const status: HttpStatus = HttpStatus.OK;

// String enum — prefer these for readability and debugging
enum Role {
  Admin = 'ADMIN',
  Editor = 'EDITOR',
  Viewer = 'VIEWER'
}

function checkAccess(role: Role): boolean {
  return role === Role.Admin || role === Role.Editor;
}

// Const enum — inlined at compile time (no runtime object)
const enum LogLevel {
  Debug = 'DEBUG',
  Info = 'INFO',
  Warn = 'WARN',
  Error = 'ERROR'
}

// Reverse mapping (numeric enums only)
enum Color {
  Red,
  Green,
  Blue
}
Color[0];        // 'Red'
Color['Red'];    // 0

Prefer string enums or union literal types (type Role = 'admin' | 'editor') over numeric enums for better debugging and type safety. Const enums produce smaller output but can't be iterated at runtime.

Decorators require "experimentalDecorators": true in tsconfig.json (legacy) or the TC39 Stage 3 decorator syntax (TS 5.0+). The examples below use the legacy syntax commonly seen in Angular, NestJS and similar frameworks.

// Class decorator
function Sealed(constructor: Function) {
  Object.seal(constructor);
  Object.seal(constructor.prototype);
}

@Sealed
class Greeter {
  greeting: string;
  constructor(message: string) {
    this.greeting = message;
  }
}

// Decorator factory — returns a decorator
function Logger(prefix: string) {
  return function (target: Function) {
    console.log(`${prefix}: ${target.name} registered`);
  };
}

@Logger('API')
class UserController { }

// Method decorator
function Log(target: any, key: string, descriptor: PropertyDescriptor) {
  const original = descriptor.value;
  descriptor.value = function (...args: any[]) {
    console.log(`Calling ${key} with`, args);
    const result = original.apply(this, args);
    console.log(`${key} returned`, result);
    return result;
  };
}

class MathService {
  @Log
  add(a: number, b: number): number {
    return a + b;
  }
}

// Property decorator
function MinLength(min: number) {
  return function (target: any, key: string) {
    let value: string;
    Object.defineProperty(target, key, {
      get: () => value,
      set: (newVal: string) => {
        if (newVal.length < min) {
          throw new Error(`${key} must be at least ${min} characters`);
        }
        value = newVal;
      }
    });
  };
}

class Account {
  @MinLength(3)
  username!: string;
}