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A Comprehensive Guide to TypeScript Decorators: How to Use Them Effectively

4 April 2023

A Comprehensive Guide to TypeScript Decorators: How to Use Them Effectively

 

TypeScript decorators are a powerful feature that allows you to add functionality to your code by decorating classes, methods, and properties with metadata. They provide a way to modify the behavior of existing code without changing its implementation. In this comprehensive guide, we will explore how to use TypeScript decorators effectively, with step-by-step instructions and code examples.

 

Overview of TypeScript Decorators

 

TypeScript decorators are a way to modify or extend the behavior of a class, method, or property. Decorators use the ‘@’ symbol followed by the decorator function name to apply the decorator to the corresponding element.

 

Decorators can be used to add metadata to classes, methods, and properties, modify the behavior of a method, or add additional functionality to a class or property. For example, decorators can be used to add validation logic to a property or method, or to add logging or caching to a method.

 

How to Use Decorators in TypeScript

 

To use decorators in TypeScript, you must enable the ‘experimentalDecoratorsflag’ in your tsconfig.json file. This flag enables the experimental decorator feature in TypeScript, allowing you to use decorators in your code.

 

{

  "compilerOptions": {

    "experimentalDecorators": true,

    ...

  },

  ...

}

Once you have enabled the ‘experimentalDecoratorsflag, you can start using decorators in your code. Decorators can be applied to classes, methods, and properties.

Class Decorators

Class decorators are applied to a class and can be used to add metadata or modify the behavior of the class.

 

 

@log

class MyClass {

  ...

}

 

In this example, the ‘@log’ decorator is applied to the ‘MyClass class. The ‘@log’ decorator can be used to log information about the class whenever it is instantiated.

Method Decorators

Method decorators are applied to a method and can be used to modify the behaviour of the method.

 

class MyClass {

  @log

  myMethod() {

    ...

  }

}

 

In this example, the ‘@log’ decorator is applied to the ‘myMethod method. The ‘@log’ decorator can be used to log information about the method whenever it is called.

Property Decorators

Property decorators are applied to a property and can be used to modify the behaviour of the property.

 

class MyClass {

  @validate

  myProperty: string;

}

 

In this example, the ‘@validate’ decorator is applied to the myProperty property. The @validate decorator can be used to validate the value of the property whenever it is set.

 

Creating Your Own Decorators

 

You can create your own decorators in TypeScript by defining a decorator function that takes a target, name, and descriptor as arguments. The target is the class or prototype that the decorator is applied to, the name is the name of the decorated property or method, and the descriptor is an object that contains information about the property or method.

function log(target: any, name: string, descriptor: PropertyDescriptor) {

  const original = descriptor.value;

  descriptor.value = function(...args: any[]) {

    console.log(`Calling ${name} with arguments ${args}`);

    const result = original.apply(this, args);

    console.log(`Result of ${name} is ${result}`);

    return result;

  }

  return descriptor;

}

 

In this example, the ‘log’ decorator function is defined to log information about a method when it is called. The target argument is the prototype of the class, the name argument is the name of the method, and the descriptor argument contains information about the method.

To apply the ‘log’ decorator to a method, you can use the ‘@’ symbol followed by the decorator function name:

class MyClass {

  @log

  myMethod() {

    ...

  }

}

This will apply the log decorator to the myMethod method, causing it to log information whenever it is called.

 

Conclusion

In this guide, we have explored how to use TypeScript decorators effectively. We have covered the basics of TypeScript decorators, including how to enable the experimental decorator feature in TypeScript, how to apply decorators to classes, methods, and properties, and how to create your own decorators.

By using TypeScript decorators, you can add metadata and functionality to your code without changing its implementation. Decorators provide a powerful way to extend the behavior of your code, making it more flexible and easier to maintain.

 

Official Documentation

For more information on TypeScript decorators, you can consult the official TypeScript documentation:

 

·        TypeScript Decorators documentation: https://www.typescriptlang.org/docs/handbook/decorators.html

 

 

 

About the author: Daniel West
Tech Blogger & Researcher for JBI Training

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