How to declare a class in TypeScript?

Declaring a class in TypeScript primarily involves using the class keyword and defining the class's constructor, properties, and methods. Here is a simple example:

typescript
class Person {
    // Properties
    name: string;
    age: number;

    // Constructor
    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }

    // Methods
    describe(): string {
        return `I am ${this.name}, I am ${this.age} years old.`;
    }
}

// Creating an object using the class
let person1 = new Person("张三", 30);
console.log(person1.describe());  // Output: I am Zhang San, I am 30 years old.

Key Components of a Class:

1. Properties: Used to define the characteristics of a class, such as name and age in the example above.
2. Constructor (constructor): A special method primarily used to initialize objects upon creation, assigning initial values to object properties.
3. Methods: Define the behavior of a class, such as the describe method which provides a description of a person.

Advanced Features of Classes:

• Inheritance: Using the extends keyword, subclasses can inherit properties and methods from the parent class.
• Access modifiers: Such as public (public), private (private), and protected (protected), used to restrict access scope.
• Static properties and methods: Using the static keyword, these properties and methods belong to the class itself rather than to instances of the class.

typescript
class Employee extends Person {
    private salary: number;  // Private property, not directly accessible from outside

    constructor(name: string, age: number, salary: number) {
        super(name, age);  // Call the parent class constructor
        this.salary = salary;
    }

    describe(): string {
        // Override the parent class method
        return `${super.describe()}, my salary is ${this.salary} yuan.`;
    }

    // Static method
    static isEmployee(obj: any): obj is Employee {
        return obj instanceof Employee;
    }
}

let employee1 = new Employee("李四", 40, 5000);
console.log(employee1.describe());  // Output: I am Li Si, I am 40 years old, my salary is 5000 yuan.
console.log(Employee.isEmployee(employee1));  // Output: true

This example demonstrates how to use classes, inheritance, access modifiers, and static methods in TypeScript.