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React Performace Guide

As we know, React re-renders the component whenever there is change in state or props. There is no magical binding or any watcher which observes the state variable and updates it , infact it re-rendering of component which updates the UI. Each render has it's own props, state, event-listeners and effects, this is very well explained by Dan.

The Problem

In below code, Parent Component has 5 child components and all are rendered again even if the change is not related to them.


Here is how performance in impacted by unnecessary re-renders of components.

The Solution

The first line of defense to avoid unnecessary re-renders is using React.memo. React.memo will skip rendering the component, and reuse the last rendered result.
Let's wrap all the child components in React.memo.

We managed to save 3 components from re-rendering on change of Name input field, but still Address Input field re-renders. React.memo only does a shallow comparison of props changes but the handleAddressChange function is recreated new for each render and since functions are compared by reference, React.memo will not able to stop it from re-rendering Input Component as it is prop is callback function, which is new for each render of Parent Component.

The way to optimize this scenario is by using useCallback hook.

Now we have re-rendering only for the components that require it, as callbacks are passed to optimized child components that rely on reference equality to prevent unnecessary renders


💡 Now suppose if we have another requirement that is to validate address and show the suggestion. Address Validation API can take some time , lets see that in action

    const isValidAddress = () => {
    console.log(`validating Address... for ${city}`);
    for (let i = 0; i < 10000; i++) {
      //some delay to simualte address validation API
    return city.length < 3 ? false : true;
<!--  markup for address validation-->
      <span> Address is {isValidAddress() ? "" : ""}</span>
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For this example , let's assume if address length is less than 3 then it's invalid otherwise it's valid.

React.useMemo (1)

Now isValidAddress() which is an expensive fn, is getting called even if we are changing the name , it is happening due re-render of component due to change in name state.
We can memoize the result of addressValidation function when we are changing any other state and only recalculate when there is change in address field.


The optimized solution with useMemo saves time of unnecessary evaluation of any expensive function. ⤵

The difference between useCallback() and useMemo() is that useCallback is to memoize/memorize the callback function that are passed to as props and useMemo is helpful in memozing the result of any expensive calculation.

Here is full code example of above concepts:

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