What does the hook numbers in the Reactjs Dev tool correspond to?

If you are a React developer, you have probably used the React DevTools extension to inspect and debug your React applications. One of the features of the DevTools is the ability to see the hook numbers associated with each component. But what do these hook numbers actually correspond to? Let’s find out!

When you open the React DevTools and inspect a component, you will see a list of hooks with corresponding numbers next to them. These numbers represent the order in which the hooks were called during the rendering process. Each time a component renders, React keeps track of the order in which the hooks are called and assigns them a unique number.

So, why is this information useful? Understanding the hook numbers can help you identify and debug issues related to the order of hooks in your components. If you notice unexpected behavior or errors, you can refer to the hook numbers to see if the hooks are being called in the expected order.

Let’s take a look at an example to better understand the concept. Consider the following component:


import React, { useState, useEffect } from 'react';

const ExampleComponent = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('Effect 1');
  }, []);

  useEffect(() => {
    console.log('Effect 2');
  }, [count]);

  return (
    
      Count: {count}
      
    
  );
};

In this example, we have two useEffect hooks. The first one runs only once when the component mounts, and the second one runs whenever the ‘count’ state changes. Now, let’s inspect this component in the React DevTools.

When you inspect the ExampleComponent in the React DevTools, you will see the hook numbers next to each hook. In this case, the first useEffect hook will have the hook number 1, and the second useEffect hook will have the hook number 2.

By observing the hook numbers, you can determine the order in which the hooks are called. In this example, the hook with number 1 will always be called before the hook with number 2, as it is defined first in the component.

It’s important to note that the hook numbers are specific to each instance of a component. If you have multiple instances of the same component, each instance will have its own set of hook numbers.

Now that you understand what the hook numbers in the React DevTools correspond to, you can use this information to debug and optimize your React applications. If you notice any unexpected behavior or issues related to hooks, checking the hook numbers can provide valuable insights.

Happy debugging!

What does the hook numbers in the Reactjs Dev tool correspond to?