When working with class components in React using TypeScript, the question is often asked whether it is necessary and mandatory to define props and state inside the constructor. The answer to this question depends on the specific needs of your component. In this blog post, we'll look at when and why to use a constructor to define props and state, as well as the pros and cons of different approaches.

Using constructors

When to use constructor:

1. State initialization based on props:

If state depends on props or if you need to perform additional logic when initializing state, a constructor is the best choice.

2. Setting initial state values:

When you want to set the initial state of a component, a constructor is the traditional way to do it.

Example:

interface IMyComponentProps {
  initialCount: number;
}

interface IMyComponentState {
  count: number;
}

class MyComponent extends React.Component {
  constructor(props: IMyComponentProps) {
    super(props);
    this.state = {
      count: props.initialCount,
    };
  }

  render() {
    return 
Count: {this.state.count}
;
  }
}

When not to use a constructor

1. Simple state initialization:

If state is not complex and does not depend on props, you can use direct state initialization without constructor.

2. No need for complex logic:

If you don't need to perform additional logic related to props or state, you can set state directly at the class level.

Example:

interface IMyComponentProps {
  message: string;
}

interface IMyComponentState {
  count: number;
}

class MyComponent extends React.Component {
  state: IMyComponentState = {
    count: 0,
  };

  render() {
    return 
Count: {this.state.count}
;
  }
}

Advantages and disadvantages of different approaches

Using the constructor:

Advantages:

• More complex initialization: Allows setting state based on props and executing additional logic before the component renders.
• Control: Allows precise control over initialization and you can easily add or modify the initialization logic.

Disadvantages:

• More code: Using constructors can add more code, which can make the component harder to read, especially if the initialization is simple.
• More complexity: Introducing additional complexity if simple initialization can be done without constructors.

Direct initialization (state) outside the constructor:

Advantages:

• Simplicity: Less code and cleaner code for simple components.
• Better readability: Components are often more readable when using direct state initialization.

Disadvantages:

• Limited flexibility: You cannot easily initialize state based on props or add complex initialization logic.

Conclusion

• Use the constructor if you need to initialize state based on props or if you have complex logic that needs to be executed before rendering the component.
• Avoid the constructor if your state initialization can be easily set directly at the class level and requires no additional logic.

Both approaches are correct and depend on the complexity of your component and specific needs. In modern React coding, many developers prefer the simpler approach of direct initialization if it meets their needs.