Decorators

A decorator is a function that runs at class definition time to wrap or modify a class, method, accessor, or field. TS 5+ ships the modern TC39 (Stage 3) form.

A Method Decorator

function log(target: Function, context: ClassMethodDecoratorContext) {
  return function (this: unknown, ...args: unknown[]) {
    console.log(`[call] ${String(context.name)}(${args.join(", ")})`);
    return target.call(this, ...args);
  };
}

class Calculator {
  @log
  add(a: number, b: number) {
    return a + b;
  }
}

new Calculator().add(1, 2);
// [call] add(1, 2)
// 3

@log runs once when the class is defined. It receives the original method and returns a replacement that logs, then delegates.

A Class Decorator

function sealed(target: Function, context: ClassDecoratorContext) {
  Object.seal(target);
  Object.seal(target.prototype);
}

@sealed
class Point {
  x = 0; y = 0;
}

Runs on the class itself — at definition time.

Common Uses

• Logging / tracing
• Validation (NestJS, class-validator)
• Dependency injection (NestJS, Inversify)
• ORM column mappings (TypeORM)

Enabling

The modern decorators are on by default in TS 5+. For the older “experimental” form (Angular, NestJS-style):

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

Note: the old and new forms have different function signatures and behavior. Pick one for a project — don’t mix.

When To Reach For Them

Decorators add a layer of magic — useful in framework code, less useful in everyday app logic. A plain higher-order function often beats a decorator for clarity.

Up Next

Enums — TS’s named-constant feature, complete with its share of gotchas.