docs: decorators & meta programming

Author: devmahmud

module-5.4-decorators-and-meta-programming.md

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+#### 5.4. Decorators and Meta-Programming
+
+Decorators and meta-programming are advanced JavaScript features that enable you to modify the behavior of classes, methods, and properties. These techniques provide powerful tools for creating reusable, clean, and maintainable code.
+
+##### Decorators in ES6
+
+Decorators allow you to attach additional behavior to classes, methods, or properties. They are often used for aspects like logging, validation, access control, and more.
+
+**Example:**
+
+Suppose you want to create a logging decorator to log method calls:
+
+```javascript
+function logMethod(target, key, descriptor) {
+  const originalMethod = descriptor.value;
+
+  descriptor.value = function (...args) {
+    console.log(`Calling method ${key} with arguments ${args}`);
+    return originalMethod.apply(this, args);
+  };
+
+  return descriptor;
+}
+
+class Calculator {
+  @logMethod
+  add(a, b) {
+    return a + b;
+  }
+}
+
+const calc = new Calculator();
+calc.add(3, 5);
+```
+
+In this example, the `logMethod` decorator logs method calls and their arguments. When the `add` method of the `Calculator` class is called, it's automatically logged.
+
+##### Reflect API
+
+The Reflect API is part of meta-programming in JavaScript, allowing you to interact with objects and classes programmatically. It can be used for tasks like intercepting object operations and creating proxy objects.
+
+**Example:**
+
+```javascript
+const handler = {
+  get(target, prop, receiver) {
+    console.log(`Getting property ${prop}`);
+    return Reflect.get(target, prop, receiver);
+  },
+};
+
+const obj = new Proxy({}, handler);
+
+obj.value = 42;
+console.log(obj.value); // Logs the property access
+
+obj.undefinedProperty; // Logs the property access
+```
+
+In this example, we use the Reflect API to intercept property access and log it.
+
+##### Use Cases
+
+Decorators and meta-programming are valuable for various tasks, including:
+
+- **Logging**: You can create logging decorators to log method calls or property accesses.
+- **Validation**: Decorators can validate input or output data.
+- **Access Control**: You can use decorators to control access to certain methods or properties.
+- **Object Operations**: Meta-programming allows you to intercept object operations and customize behavior.
+
+#### Benefits
+
+- Decorators and meta-programming enhance code readability and maintainability by separating concerns.
+- They offer powerful tools for creating reusable and clean code with additional behavior attached to classes, methods, or properties.
+
+Mastering decorators and meta-programming can greatly improve your ability to create maintainable and flexible code. These techniques are valuable for implementing cross-cutting concerns and improving code organization.