How to Compare Oldvalues and Newvalues on React Hooks Useeffect?

React Hooks introduced a new way of managing state and side effects in functional components. One of the most commonly used hooks is useEffect, which allows us to perform side effects in our components. However, a common question that arises when using useEffect is how to compare the old and new values of a dependency to determine if the effect should be executed.

By default, useEffect runs after every render of the component. If you provide an array of dependencies as the second argument to useEffect, it will only run the effect if any of the dependencies have changed since the last render. However, this comparison is done using strict equality (===), which may not work as expected for objects and arrays.

Using the JSON.stringify method

One way to compare objects and arrays is by converting them to JSON strings and comparing the strings. This can be done using the JSON.stringify method. Here’s an example:

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

const MyComponent = ({ data }) => {
  const [oldData, setOldData] = useState(null);

  useEffect(() => {
    if (oldData !== null && JSON.stringify(oldData) !== JSON.stringify(data)) {
      // Perform side effect here
    }
    setOldData(data);
  }, [data]);

  return (
    // JSX code here
  );
};

export default MyComponent;

In the above example, we store the old data in a state variable called oldData. Inside the useEffect hook, we compare the oldData with the new data using JSON.stringify. If the string representations of the old and new data are not equal, we perform the desired side effect.

Using a custom comparison function

If you need more control over the comparison logic, you can provide a custom comparison function as the second argument to useEffect. This function should return true if the dependencies are considered equal and false otherwise. Here’s an example:

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

const MyComponent = ({ data }) => {
  const [oldData, setOldData] = useState(null);

  useEffect(() => {
    const areEqual = (prevData, newData) => {
      // Custom comparison logic here
    };

    if (oldData !== null && !areEqual(oldData, data)) {
      // Perform side effect here
    }
    setOldData(data);
  }, [data]);

  return (
    // JSX code here
  );
};

export default MyComponent;

In the above example, we define a custom comparison function called areEqual. Inside the useEffect hook, we call this function to determine if the old and new data are considered equal. If the function returns false, we perform the desired side effect.

By using either the JSON.stringify method or a custom comparison function, you can compare the old and new values of dependencies in useEffect and execute the effect only when necessary.

Remember to always consider the performance implications of your comparison logic, especially if you’re dealing with large objects or arrays. In some cases, it may be more efficient to perform a shallow comparison or use a library like lodash’s isEqual.