Prefer ECMAScript Features to TypeScript Features

Overview

TypeScript includes features like enum and namespace that are specific to TypeScript and don't exist in JavaScript. Prefer standard ECMAScript features when possible - they're more portable, better understood by the JavaScript ecosystem, and won't lock you into TypeScript-specific patterns.

This doesn't mean avoid TypeScript entirely - use its type system fully. But for runtime features, prefer standard JavaScript.

When to Use This Skill

• Choosing between TypeScript and ECMAScript features
• Writing portable code that might be used without TypeScript
• Considering enum, namespace, or parameter properties
• Targeting multiple JavaScript environments
• Writing library code for broad consumption

The Iron Rule

Prefer standard ECMAScript features over TypeScript-specific runtime features. Use TypeScript for types, JavaScript for runtime behavior.

Detection

Watch for TypeScript-specific runtime features:

// RED FLAGS - TypeScript-specific features
enum Status { Active, Inactive }  // TypeScript enum
namespace MyLib { }               // TypeScript namespace
class Foo { constructor(public x: number) {} }  // Parameter properties

Enums: Use Const Objects Instead

// TypeScript enum - generates runtime code
enum Status {
  Active = 'ACTIVE',
  Inactive = 'INACTIVE',
}
// Generates: var Status = { Active: 'ACTIVE', ... }

// BETTER: Const object with type
const Status = {
  Active: 'ACTIVE',
  Inactive: 'INACTIVE',
} as const;

type Status = typeof Status[keyof typeof Status];
// Status = 'ACTIVE' | 'INACTIVE'

// Usage
function setStatus(status: Status) { }
setStatus(Status.Active);  // OK
setStatus('ACTIVE');       // OK
setStatus('INVALID');      // Error!

Namespaces: Use ES Modules Instead

// TypeScript namespace
namespace Utils {
  export function formatDate(d: Date): string {
    return d.toISOString();
  }
  export const PI = 3.14159;
}

// BETTER: ES Module
// utils.ts
export function formatDate(d: Date): string {
  return d.toISOString();
}
export const PI = 3.14159;

// usage.ts
import { formatDate, PI } from './utils';

Parameter Properties: Be Explicit

// TypeScript parameter properties
class Person {
  constructor(
    public name: string,
    private age: number,
    protected id: string
  ) {}
}

// BETTER: Explicit declarations
class Person {
  name: string;
  private age: number;
  protected id: string;
  
  constructor(name: string, age: number, id: string) {
    this.name = name;
    this.age = age;
    this.id = id;
  }
}

When TypeScript Features Are OK

// Type-only features are fine - no runtime impact
type User = { name: string };  // TypeScript type
interface Config { }           // TypeScript interface
type ID = string;              // Type alias

// These compile away completely!

Pressure Resistance Protocol

When choosing features:

1. Ask: Is this type-only? TypeScript types are great
2. Check runtime impact: Does this generate special code?
3. Consider portability: Will this work without TypeScript?
4. Prefer standards: ECMAScript features work everywhere
5. Document exceptions: If you must use TS features, explain why

Red Flags

Common Rationalizations

"Enums are more ergonomic"

Reality: Const objects with as const provide the same type safety with standard JavaScript.

"Namespaces organize code"

Reality: ES modules are the standard. Use files and imports for organization.

"Parameter properties save typing"

Reality: They're non-standard. Explicit properties are clearer and more portable.

Quick Reference

The Bottom Line

Use TypeScript for its type system, but prefer standard ECMAScript features for runtime code. This makes your code more portable, better understood, and future-proof.

Reference

• Effective TypeScript, 2nd Edition by Dan Vanderkam
• Item 72: Prefer ECMAScript Features to TypeScript Features