Check new and old props in componentdidmount

In React, componentDidMount() is a lifecycle method that is called after the component has been rendered to the DOM. It's a good place to put code that needs to run after the component has been mounted, such as setting up event listeners or making API requests.

However, when it comes to checking for new and old props, componentDidMount() is not the best place to do it. This is because componentDidMount() is called only once, after the initial render, and it doesn't get called again when the props change.

Instead, you can use the componentDidUpdate() lifecycle method, which is called after the component has been updated (i.e., after the props have changed). Here's an example:

class MyComponent extends React.Component {
  componentDidUpdate(prevProps, prevState) {
    if (this.props.newProp!== prevProps.newProp) {
      // do something with the new prop
    }
  }
}

In this example, componentDidUpdate() is called after the component has been updated, and it receives two arguments: prevProps and prevState. prevProps is an object that contains the previous props, and prevState is an object that contains the previous state.

You can use prevProps to compare the new props with the old props, and do something with the new prop if it's different.

Alternatively, you can use the shouldComponentUpdate() method to check if the props have changed, and if so, update the component accordingly. Here's an example:

class MyComponent extends React.Component {
  shouldComponentUpdate(nextProps, nextState) {
    if (nextProps.newProp!== this.props.newProp) {
      return true;
    }
    return false;
  }
}

In this example, shouldComponentUpdate() is called before the component is updated, and it receives two arguments: nextProps and nextState. nextProps is an object that contains the new props, and nextState is an object that contains the new state.

If the props have changed, shouldComponentUpdate() returns true, and the component is updated. If the props haven't changed, shouldComponentUpdate() returns false, and the component is not updated.