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Using the 'reflect' Keyword in TypeScript

The 'reflect' keyword in TypeScript is not a keyword in the classical sense, but rather a part of the 'reflect-metadata' library, which is a polyfill for the Metadata Reflection API. This API allows you to add, read, and remove metadata from a class, method, or property.

What is Metadata Reflection?

Metadata reflection is a feature that allows you to add additional information to a class, method, or property, and then reflect on that information at runtime. This can be useful for a variety of purposes, such as dependency injection, validation, and logging.

Using the 'reflect-metadata' Library

To use the 'reflect-metadata' library, you need to install it using npm or yarn:


npm install reflect-metadata

Once installed, you can import the library and use its functions to add, read, and remove metadata.

Adding Metadata

You can add metadata to a class, method, or property using the 'defineMetadata' function:


import 'reflect-metadata';

class MyClass {
  myMethod() {}
}

defineMetadata('my-metadata', 'Hello, world!', MyClass.prototype, 'myMethod');

Reading Metadata

You can read metadata from a class, method, or property using the 'getMetadata' function:


import 'reflect-metadata';

class MyClass {
  myMethod() {}
}

defineMetadata('my-metadata', 'Hello, world!', MyClass.prototype, 'myMethod');

const metadata = getMetadata('my-metadata', MyClass.prototype, 'myMethod');
console.log(metadata); // Output: Hello, world!

Removing Metadata

You can remove metadata from a class, method, or property using the 'deleteMetadata' function:


import 'reflect-metadata';

class MyClass {
  myMethod() {}
}

defineMetadata('my-metadata', 'Hello, world!', MyClass.prototype, 'myMethod');

deleteMetadata('my-metadata', MyClass.prototype, 'myMethod');

const metadata = getMetadata('my-metadata', MyClass.prototype, 'myMethod');
console.log(metadata); // Output: undefined

Example Use Case: Dependency Injection

One common use case for metadata reflection is dependency injection. You can use metadata to specify the dependencies of a class, and then use a dependency injection container to resolve those dependencies.


import 'reflect-metadata';

class MyClass {
  constructor(private myService: MyService) {}
}

defineMetadata('dependencies', [MyService], MyClass);

class MyService {}

const container = new Container();
container.register(MyClass);
container.register(MyService);

const myClass = container.resolve(MyClass);
console.log(myClass.myService); // Output: MyService {}

Conclusion

The 'reflect-metadata' library provides a powerful way to add, read, and remove metadata from classes, methods, and properties in TypeScript. This can be useful for a variety of purposes, such as dependency injection, validation, and logging.

Frequently Asked Questions

Q: What is metadata reflection?

A: Metadata reflection is a feature that allows you to add additional information to a class, method, or property, and then reflect on that information at runtime.

Q: How do I use the 'reflect-metadata' library?

A: You can use the 'reflect-metadata' library by importing it and using its functions to add, read, and remove metadata.

Q: What is the difference between 'defineMetadata' and 'getMetadata'?

A: 'defineMetadata' is used to add metadata to a class, method, or property, while 'getMetadata' is used to read metadata from a class, method, or property.

Q: Can I use metadata reflection with other libraries?

A: Yes, metadata reflection can be used with other libraries, such as dependency injection containers and validation libraries.

Q: Is metadata reflection supported in all browsers?

A: Metadata reflection is supported in most modern browsers, but it may not be supported in older browsers.

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