reflect-metadata

import "reflect-metadata"; // Must be at the top of your entry file. const classMetadataKey = Symbol("classMetadata"); const methodMetadataKey = Symbol("methodMetadata"); const propertyMetadataKey = Symbol("propertyMetadata"); // Custom decorator to define class-level metadata function ClassMetadata(value: string) { return function <T extends { new (...args: any[]): {} }>(constructor: T) { Reflect.defineMetadata(classMetadataKey, value, constructor); }; } // Custom decorator to define method-level metadata function MethodMetadata(value: string) { return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) { Reflect.defineMetadata(methodMetadataKey, value, target, propertyKey); }; } // Custom decorator to define property-level metadata function PropertyMetadata(value: string) { return function (target: any, propertyKey: string | symbol) { Reflect.defineMetadata(propertyMetadataKey, value, target, propertyKey); }; } @ClassMetadata("MyClassValue") class MyService { @PropertyMetadata("MyPropertyValue") public someProperty: string = "hello"; constructor() { // console.log("MyService instantiated."); } @MethodMetadata("MyMethodValue") public doSomething(arg: string): string { return `Doing something with ${arg}`; } } // --- Reflection to retrieve metadata --- // Get class metadata const classMeta = Reflect.getMetadata(classMetadataKey, MyService); console.log(`Class Metadata: ${classMeta}`); // Get property metadata (on the prototype for instance properties/methods) const propertyMeta = Reflect.getMetadata(propertyMetadataKey, MyService.prototype, "someProperty"); console.log(`Property Metadata (someProperty): ${propertyMeta}`); // Get method metadata const methodMeta = Reflect.getMetadata(methodMetadataKey, MyService.prototype, "doSomething"); console.log(`Method Metadata (doSomething): ${methodMeta}`); // Example of built-in TypeScript design-time type metadata (requires tsconfig.json: "emitDecoratorMetadata": true) // For a method parameter's type function logParameterType(target: any, propertyKey: string, parameterIndex: number) { const paramTypes = Reflect.getMetadata("design:paramtypes", target, propertyKey); if (paramTypes) { console.log(`Parameter types for method '${String(propertyKey)}':`, paramTypes.map(t => t.name)); } else { console.log(`No design:paramtypes metadata found for '${String(propertyKey)}'. Ensure emitDecoratorMetadata is true.`); } } class AnotherService { public greet(@logParameterType name: string, @logParameterType age: number): string { return `Hello ${name}, you are ${age} years old.` } } const anotherService = new AnotherService(); anotherService.greet("Alice", 30);