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Using TypeScript Type Annotations with Class Properties

TypeScript is a statically typed language that allows developers to add type annotations to their code. When working with classes, type annotations can be used to specify the types of properties, methods, and their return types. In this article, we will explore how to use TypeScript type annotations with class properties.

Basic Syntax

The basic syntax for adding type annotations to class properties in TypeScript is as follows:


class MyClass {
  myProperty: string;
}

In this example, the `myProperty` property is annotated with the `string` type, indicating that it can only hold string values.

Access Modifiers

TypeScript also supports access modifiers for class properties, which can be used to control access to the properties. The three access modifiers available in TypeScript are:

  • `public`: The property can be accessed from anywhere.
  • `private`: The property can only be accessed within the same class.
  • `protected`: The property can only be accessed within the same class and its subclasses.

Here is an example of using access modifiers with class properties:


class MyClass {
  public myPublicProperty: string;
  private myPrivateProperty: string;
  protected myProtectedProperty: string;
}

Readonly Properties

TypeScript also supports readonly properties, which can be used to create properties that can only be assigned a value during initialization. Here is an example of using readonly properties:


class MyClass {
  readonly myReadonlyProperty: string;
  constructor(myValue: string) {
    this.myReadonlyProperty = myValue;
  }
}

Optional Properties

TypeScript also supports optional properties, which can be used to create properties that may or may not be present. Here is an example of using optional properties:


class MyClass {
  myOptionalProperty?: string;
}

Default Values

TypeScript also supports default values for class properties. Here is an example of using default values:


class MyClass {
  myProperty: string = 'default value';
}

Getters and Setters

TypeScript also supports getters and setters for class properties. Here is an example of using getters and setters:


class MyClass {
  private _myProperty: string;

  get myProperty(): string {
    return this._myProperty;
  }

  set myProperty(value: string) {
    this._myProperty = value;
  }
}

Conclusion

In this article, we explored how to use TypeScript type annotations with class properties. We covered the basic syntax, access modifiers, readonly properties, optional properties, default values, and getters and setters. By using type annotations with class properties, developers can create more robust and maintainable code.

Frequently Asked Questions

Q: What is the purpose of type annotations in TypeScript?

A: The purpose of type annotations in TypeScript is to specify the types of variables, properties, and function parameters, which helps to catch type-related errors at compile-time rather than runtime.

Q: How do I add type annotations to class properties in TypeScript?

A: To add type annotations to class properties in TypeScript, you can use the syntax `propertyName: type;` after the property name.

Q: What are access modifiers in TypeScript?

A: Access modifiers in TypeScript are used to control access to class properties and methods. The three access modifiers available in TypeScript are `public`, `private`, and `protected`.

Q: How do I create readonly properties in TypeScript?

A: To create readonly properties in TypeScript, you can use the `readonly` keyword before the property name.

Q: How do I create optional properties in TypeScript?

A: To create optional properties in TypeScript, you can use the `?` symbol after the property name.

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