ReactJS: memo and useMemo for Memorized Computations

Anton Ioffe - September 1st 2023 - 31 minutes read

ReactJS Memoization: The Role of memo and useMemo.

In an environment where efficient resource utilization is crucial, we want our web applications to take the least time possible to render while consuming the least resources. One of the ways ReactJS empowers developers to create more efficient applications is through memoization, introducing us to the React.memo and useMemo hooks. These two functionalities primarily target one of the costliest parts of applications: complex computations.

The premise behind memoization in ReactJS, or any other context, is simple: expensive function calls should not be re-computed when given the same input. In other words, if a function's outputs solely depend on its inputs, we can store these outputs as soon as they are computed. When the function is called again with the same set of inputs, we can just use the stored output, thereby saving computational resources and time. This strategy benefits most in scenarios where complex or expensive computations recur with identical parameters.

React.memo is a higher order component which wraps around a functional component and prevents it from re-rendering if its props do not change. A simple use of React.memo would look like this:

const MyComponent = React.memo(function MyComponent(props) {
   /* render using props */

Here, MyComponent will only re-render when its props change.

On the other hand, useMemo is a React hook providing an optimized way to compute and memorize values derived from expensive functions. useMemo accepts two parameters: a function and a dependency array, and it will only re-execute the function if the values in the dependency array change. Here is a brief example:

const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

In this variant, computeExpensiveValue(a, b) only re-executes when a or b changes.

While both React.memo and useMemo serve to optimize our applications, they also come with their own complexities. They can increase the code's complexity and potentially lead to more challenging debugging scenarios. For this reason, consider if the optimization benefits outweigh the added complexity before applying these techniques.

As JavaScript developers, we always strive for better performance and efficient resource use. Is the overhead of memoization worth the benefits in your current project? Could the use of React.memo and useMemo result in an observable enhancement in your web application's performance? Moreover, can these benefits justify any potential increase in code complexity? As always, when it comes to optimization, think critically, analyze thoroughly, and choose wisely.

Shedding Light on Memorization in ReactJS.

In the dynamic and complex realm of web development, ReactJS has established itself as a paramount library for crafting fluid user interfaces. Amongst the library's wealth of features, memorization has garnered attention due to its potential contributions to performance enhancement. We're going to explore memorization in ReactJS, paying special attention to the React.memo and useMemo hooks.

Understanding Memorization

In coding terminology, memorization refers to an optimization technique for processing computation-heavy functions. It achieves this by caching the results of these taxing computations. In simpler terms, it involves storing the outcomes of costly processes, allowing for their subsequent utilization when needed, thus alleviating the need for repeated taxing computations. ReactJS caters to the demand for memorization using the React.memo and useMemo hooks. The first allows the creation of memorized components, while the latter is responsible for generating memorized values.

Here is a JavaScript illustration to elucidate this concept:

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

const ExpensiveComponent = React.memo(({ compute, value }) => {
    const computedValue = compute(value);
    return <div>{'Computed value: ' + computedValue}</div>;

const ParentComponent = () => {
    const [value, setValue] = useState(0);

    const compute = useMemo(() => {
        let result = 0;
        // Execute some heavy computation here
        for(let i = 0; i < 10000000; i++){
        // Return the computed value afterwards 
        return result;
    }, [value]);

    return (
            <button onClick={() => setValue(value + 1)}>
                'Increment value'
            <ExpensiveComponent compute={compute} value={value} />

In this piece of code, each rerender of the ParentComponent incites the rendering of a fresh ExpensiveComponent instance. However, due to React.memo, if the prop values remain unchanged from the last render, it avoids recomputation. This structure proves beneficial for performance enhancement.

However, heed must be paid to the fact that while memorization can greatly contribute to performance improvements, it isn't a be-all and end-all solution for performance issues. In fact, the overuse or misuse of memorization techniques can increase memory consumption due to the continuous caching of computed results and their raw data.

Memoization: The Good, The Bad, and The Misused

Just as the judicious use of memorization can reap significant performance improvements, its misuse can result in unnecessary computations and excessive memory use. For instance, take a look at this common misuse scenario:

import React, { useMemo } from 'react';

const ComponentWithMisuse = ({ array }) => {
    const largeArray = useMemo(() => [...Array(1000000).keys()], [array]);
    return <div>{'Array length: ' + array.length}</div>;

In this code snippet, largeArray always contains a million elements, regardless of the size of the input array. This not only wastes memory but might slow down your app due to the unnecessary creation of such a large array.

Contrarily, here is a demonstration of how memoization should be correctly implemented:

import React, { useMemo } from 'react';

const ComponentWithUse = ({array}) => {
    const largeArray = useMemo(() => [...Array(array.length).keys()], [array]);
    return <div>{'Array length: ' + largeArray.length}</div>;

In this example, largeArray only contains as many elements as the input array, and it will only be recalculated when the array changes – showcasing an efficient use of useMemo.

But, understanding and implementing memoization correctly involves a different skill set. It's critical to avoid the trap of thinking that you can simply use memoization to solve all performance issues in your application. Remember to conduct comprehensive performance testing both pre and post adding memoization.

Ask yourself these questions: How often do the heavy functions that could potentially benefit from memoization run? Will the performance benefits of memoization offset the extra memory used in your specific application scenario? The answers to these questions should guide your decision on whether to implement memoization or consider alternative performance optimization approaches.

In summary, when executed correctly, memorization can significantly enhance the performance of your application. But remember, it is not an overarching solution to all your performance issues. Each implementation of memoization should be thought out carefully, considering the unique requirements and circumstances of your application.

Exploring the Distinctiveness of Hooks: memo, useMemo, useRef, useEffect, and useCallback.

When working with ReactJS, hooks prove to be indispensable. They allow us to take advantage of state and various other React features more effortlessly, all without having to write a class component. Five significant hooks provided by React, each with distinct roles, are memo(), useMemo(), useRef(), useEffect(), and useCallback(). These specific hooks, each having unique impact, play crucial roles in sustaining performance and readability. Our journey begins as we delve deeper into each, identifying their exclusive characteristics and application.

React.memo(): Optimizing Functional Components

React.memo() is a higher-order component that memorizes the previously rendered output, then repurposes this if the props are identical. Put simply, it insulates your component from needless re-renders when incoming props retain their initial state. The optimization in rendering large lists can come as a significant benefit.

But, for clarity, React.memo() should not be misconstrued as JavaScript memoization. JavaScript memoization involves storing the result of an expensive function call and reusing this cached outcome when the same inputs arise again. Contrastingly, React.memo() functions as a mechanism to prevent a component from re-rendering based on prop comparison.

Consider this TypeScript example where React.memo() halts the DataDisplay component from re-rendering as long as the jsonData prop retains its value.

const DataDisplay: React.FC<{jsonData: JSONType}> = 
    React.memo(({ jsonData }) => {
    /* Heavy rendering based on jsonData */

// In parent component
<DataDisplay jsonData={largeJsonData} />;

However, one must be cautious to avoid overusing React.memo(), particularly in the absence of a performance issue. Unnecessary memoization may lead to memory overuse, causing complications rather than solving them.

useMemo: Minimizing Expensive Computations

To memorize expensive computational values, we employ the useMemo() hook. This hook provides a memoized value that only recalculates when changes occur in its dependencies. Refer to the given TypeScript example.

const itemCount: number = useMemo(() => expensiveComputation(items), [items]);

Here, useMemo() guarantees that the itemCount does not recompute until items undergo alterations. Misusing useMemo can degrade performance, hence it's best not to use it with inexpensive synchronous computations or values that don't change frequently.

useRef: Holding Mutable Values Across Rerenders

For the creation of a mutable variable that maintains its value over multiple renders, useRef() comes into play. It also serves in accessing the DOM directly, as demonstrated in this TypeScript example:

const myDiv: React.RefObject<HTMLDivElement> = useRef(null);

// In render
<div ref={myDiv} />

// Accessing the div directly

Nonetheless, an all-too-common pitfall involves misusing useRef to incite re-renders or contain state-like values. It's not a substitute for hooks such as useState or useEffect.

useEffect: Executing Side Effects

To perform side effects in your functional components, useEffect() comes into play. By default, it runs post every render, but you can instruct it to only run when specific values change by declaring a dependencies array.

useEffect(() => {
        .then(data => setItems(data));
}, [inputValue]);

In this snippet, the code fetches items whenever inputValue changes. A cardinal rule with useEffect is that it doesn't halt your component from rendering, hence avoid drawn-out operations within it.

useCallback: Memorizing Function Instances

useCallback() grants memoization of a function instance, a boon in scenarios where optimized child components dependent on reference equality aim to curb unnecessary renders.

const memoizedCallback = useCallback(() => {
    // Some prop/ state dependent logic
}, [prop, state]);

This line of code will yield a memoized function variant that changes solely if prop or state changes. However, useCallback might be an overkill if the function isn't used as a dependency of React.memo(), useEffect(), or useMemo().

The hooks mentioned each bring unique advantages toward refining the performance, readability, and modularity of your TypeScript-powered React code. Yet, misuse or overuse can lead to an unwelcome surge in complexity. Spotting appropriate occasions to utilize these hooks will aid in crafting clean, optimized code. Always ponder whether calculating a value could be done inside a useMemo to avoid needless re-computation upon every render, or whether a useCallback would better serve in preventing unnecessary renders in certain child components. Balanced judgments, guided by your requirements, will steer efficient decision-making.

So, have you encountered any scenarios where memoization led to improved performance? Or, perhaps you've experienced memory overuse due to unnecessary memoization? Reflect on these questions and continue experimenting with these hooks to find the perfect balance for your application's needs.

The Pillars of Memorization: Understanding memo and useMemo.

In the world of ReactJS, the concepts of memoization are implemented using two powerful strategies: memo and useMemo. These play a pivotal role in optimizing and enhancing the performance of your application, as we'll explore in this and upcoming sections.

Getting Started: What are memo and useMemo

Let's demystify what memo and useMemo truly are.

React.memo() is a higher-order function that permits the optimization of the render process of functional components by isolating them. Essentially, memo caches the last rendered output of a functional component and reuses it if the props haven't changed in subsequent re-renders.

useMemo(), on the other hand, is a hook employed to memorize computed values. Ordinarily, all components and their local variables are recreated during each re-render. However, with useMemo, you can direct React to retain the value. The value is only recreated when its dependencies change.

import React from 'react';

type Props = {
  value: number;

const MyComponent: React.FC<Props> = ({ value }) => {
  const computedValue = React.useMemo(() => value ** 2, [value]);

  // Remaining part of the component

In this case, computedValue will only recalculate if value changes.

When Should You Use memo or useMemo

Although both memo and useMemo have immense benefits, using them inappropriately can lead to unintentional performance bottlenecks.

React.memo() should be considered only when:

  • The component is re-rendered frequently.
  • A considerable amount of time is spent during the re-rendering process.
  • Re-renders occur even when the props are unchanged.

Notably, React.memo() only checks for prop changes. If a component relies on a global state or context, it will only re-render when its props change, not when the global state changes.

import React from 'react';

type Props = {
  prop: any;

const MyComponent: React.FC<Props> = (props) => {
  // Component's implementation

export default React.memo(MyComponent);

Conversely, useMemo() is recommended when:

  • Your application's performance is highly sensitive.
  • A certain function includes complex computations.
  • Not all functional dependencies change with every render.

Misunderstandings with useMemo usage can hinder its ability and may degrade performance. For example:

import React from 'react';

type Props = {
  values: number[];

const MyComponent: React.FC<Props> = ({ values }) => {
  const sortedValues = React.useMemo(() => [...values].sort(), []);
  // Other part of the component

In this case, the dependency array passed to useMemo is empty. This means that sortedValues only calculates once and never updates, even when values changes. This can lead to unforeseen bugs in your application.

memo and Pure Functional Components

It's worth mentioning the improper usage of memo with Pure Functional Components. In React, Functional Components are considered 'pure' if they always produce the same output given identical props or state.

When memo is used with Pure Functional Components, there are pitfalls to be mindful of. For example, if a Functional Component is a child of another component and inherits some props from the parent, the memoized component will not re-render when the parent state changes. This is because memo prevents the re-rendering of the component unless its props change.

To illustrate, here is a common mistake to avoid when using memo:

import React from 'react';

// Antipattern: Memoizing a component without large frequent rerenders
type Props = {
  title: number;

const NonRerenderingComponent: React.FC<Props> = React.memo(({ title }) => {
  return <h1 className='non-rerendering-component'>{title}</h1>;

In this instance, using memo with the NonRerenderingComponent, which does not frequently re-render and has minimal render time, is unnecessary. The overhead of memoization could offset its benefits.

As a best practice, bear in mind that while memoization provides a performance boost, it can come at the cost of memory or complexity. Understanding these subtleties can help you avoid potential obstacles, empowering your application to perform optimally.

Beyond the performance benefits, memory usage and complexity can increase when memo and useMemo are applied globally. As they hold copies of functional elements or computations in a cache-like behavior, this increases memory usage which should be taken into account. Balancing this aspect with the gains in speed and re-rendering optimization is crucial in creating a performant ReactJS application.

Now, it's time for a little self-reflection - are you thoughtfully utilizing memo and useMemo in your ReactJS applications? Considering the performance, memory usage and complexity trade-offs, could this new knowledge prompt further optimization in your code?

useMemo: The Key to Optimizing Complex Computations and Rendering.

React's useMemo hook can be an essential tool to solve performance issues related to complex computations and rendering. It holds the potential to significantly impact the performance of a React application by memorising the 'expensive' computations and allowing us to reuse these memoized results, hence reducing computationally expensive operations' costs in rerendering scenarios.

While useMemo is a potent tool, it's important to strike a balance in its use. Overusing useMemo in trivial conditionals or calculations that do not rely on component updates won't provide much performance gain and may instead increase the memory footprint, creating a performance bottleneck. Therefore, consider only using useMemo for computationally expensive operations.

Let's dive deeper into useMemo and understand how it works with some TypeScript examples.

import React, { useMemo } from 'react';

type PropType = {
  list: number[];

const ListItem = React.memo((props: PropType) => {
  const computedValue = useMemo(() => expensiveComputation(props.list), [props.list]);

  return (

// expensive computation task
function expensiveComputation(list: number[]): number {

  return list.reduce((a, b) => a + b, 0); // example of a computation task

In the example above, the computedValue gets cached after the first render. Subsequent renders will reuse this cached value, and the expensiveComputation function will only be invoked again if props.list changes. useMemo thus helps in reducing the computation cost of re-rendering the ListItem component.

Now let's move on to discussing parameter passing. More often than not, these computations depend on certain variables or props in your component. useMemo allows you to pass these dependencies as an array in the second parameter.

It's crucial to correctly assess and declare these dependencies for useMemo to perform optimally. Not doing so may lead to unexpected behavior and stale data.

import React, { useMemo } from 'react';

type PropType = {
  listOne: number[];
  listTwo: number[];

const ListItem = React.memo((props: PropType) => {
  const computedValue = useMemo(() => expensiveComputation(props.listOne, props.listTwo), [props.listOne, props.listTwo]);

  return (

// expensive computation task
function expensiveComputation(listOne: number[], listTwo: number[]): number {

  return listOne.concat(listTwo).reduce((a, b) => a + b, 0); // example of a computation task

In this example, we have two lists, listOne and listTwo, and we need to perform our computation considering both. We pass the lists as dependencies to the useMemo hook and the function expensiveComputation is called any time either of these lists changes.

As developers, one is often forced to contemplate: Is useMemo worth it? The answer in most cases is not straightforward. useMemo could be an overkill for trivial computations. Plus, arbitrarily wrapping all calculations with useMemo can make the code harder to read.

Remember, useMemo comes with its own overhead cost of needing to maintain the cache in memory. Its misuse can lead to more memory consumption, which might challenge the performance of the application.

That said, when dealing with computations that can prove to be expensive performance-wise and can be invoked several times with the same parameters, useMemo can result in significant performance optimization, enhancing overall user experience.

Is there a computation or rendering task in your current project that useMemo could help optimize? How can you ensure that you are using this hook responsibly, considering its pros and cons? Ponder over these questions as you refactor your components to use useMemo.

Achieving Maximum Efficiency with Parameter Passing in useMemo.

Understanding Parameter Passing in useMemo

For supercharging the efficiency of useMemo, a deep-dive into its parameter passing mechanism is essential. The useMemo hook accepts two parameters:

  1. A function that embodies the result that needs to be memoized.
  2. An array of dependencies that the function relies on to execute.

How you approach these two parameters determines how much value useMemo can bring to your applications. Let's explore more with the help of a TypeScript example showcasing the implementation of useMemo, where we compute the total cost of items in a shopping cart:

let totalCost = useMemo((): number => calculateTotalCost(cartItems), [cartItems]);

In the above code, the useMemo hook acquires a memoized version of the calculateTotalCost function. It only re-triggers this function when there are updates to the cartItems array.

Caution with the Dependency Array

Paying careful attention to the dependency array is of vital importance. A forgotten dependency can provoke the hook to incorrectly repurpose an outdated computation, leading to elusive bugs. To better comprehend this, let's look at the following flawed TypeScript code:

let totalCost = useMemo((): number => calculateTotalCost(cartItems, discountCode), [cartItems]); // discountCode dependency missing from the array

In this faulty example, alterations to the discountCode value will not be considered. This causes useMemo to reapply an outdated computation and consequently give an incorrect total cost.

Overuse of useMemo is another potential pitfall, as it may stress your application's performance and bloat the code complexity. This prompts the question: Are you judicious with your application of useMemo?

Efficient Parameter Passing in useMemo

The order of parameters in the functions that accept multiple parameters cannot significantly influence the performance of useMemo. Irrespective of their order, React's useMemo hook will simply check if values of dependencies have changed between renderings. The order of the parameters oriented based on change frequency will not affect its computations in any way.

Consider the calculateTotalCost function:

let totalCost = useMemo((): number => calculateTotalCost(cartItems, discountCode), [cartItems, discountCode]);

In the above snippet, irrespective of whether cartItems is placed before discountCode, useMemo just checks if any of these dependencies have changed between renderings.

Misuse of useMemo: An Example

A common misuse of useMemo is utilizing it out of the rendering method or functional component scope. Let's glance at an example:

let expensiveValue = useMemo(() => doExpensiveCalculation(input), [input]);

function Component(props) {
  // ...
  // Using expensiveValue in our component
  return <div>The calculated value is {expensiveValue}</div>

Even though you might think this is fine, it violates React's Rules of Hooks. Hooks should be called only within the functional components or custom Hooks. Using useMemo outside these scenarios can lead to unpredictable results.

React.memo & Its Correlation with useMemo

useMemo aids in optimizing computation-heavy operations, and React.memo complements it by optimizing React components. React.memo memorizes the output of a functional component and skips unnecessary re-renders if the prop values don't change.

Let's delve into a TypeScript example for clarity:

interface Props {
  cartItems: Item[];
  discountCode: string;

const TotalCost = React.memo(({cartItems, discountCode}: Props) => {
  const totalCost = calculateTotalCost(cartItems, discountCode);
  return (
    <div>The total cost is {totalCost}</div>
}, function areEqual(prevProps, nextProps) {
    // defining the comparison function to maximize efficiency.
    return prevProps.cartItems.length === nextProps.cartItems.length && prevProps.discountCode === nextProps.discountCode;

In the example above, the TotalCost component uses React.memo to prevent unnecessary re-renders when the cartItems and discountCode props remain unaltered.

On the whole, useMemo and React.memo bring increased performance to your React applications when used wisely. Keep in mind, the importance of the dependency array and the significance of correctly understanding the order of parameters can greatly affect the efficient use of useMemo. As with all tools, incorrect usage might lead to added complexity and dilemmas. Always weigh the pros and cons in order to devise the best approach for your specific case.

Adopting Effective Memorization Practices in React.

In the realm of modern web development, proficient utilization of techniques like React.memo and useMemo can often facilitate effective memoization. This, in turn, can abate unnecessary re-rendering of components, conserve memory usage, and reinforce application performance. However, an imprudent overuse or misuse of these methodologies can also introduce complications and pitfalls.

Before delving further into React.memo and useMemo, let's briefly explore the concept of PureComponent.

React.memo vs PureComponent

In the traditional domain of class components in React, PureComponent primarily finds its usage. It performs a shallow comparison of the current and new props to determine when the component requires a re-render. As contemporary React development seems to incline towards functional components, React.memo emerges as the PureComponent equivalent for such components.

Note: React.memo and PureComponent essentially serve the same purpose of minimizing unnecessary renders. Therefore, if subsequent sections delve into PureComponent, keep in mind that using these two strategies together introduces redundancy.

// TypeScript example
class MyComponent extends React.PureComponent {
  render() {
    return <h1>Hello, {}</h1>;

const MyComponent: React.FC<Props> = React.memo((props: Props) => {
  // Functional component code

Please note that the principal differentiator between these two lies in their syntax and applicable context. Use PureComponent for class components and React.memo for functional ones.

Having said that, PureComponent won't be a focus within our discussion, with no further details about this being provided here.

Let's divert our attention to discern between useEffect and useMemo.

useMemo vs useEffect

In its essence, useMemo is a hook that returns memoized values. This specifically means that it allows for the optimization of heavy computational operations by storing their result, only recalculating whenever its dependencies change. useEffect has a broader utility, hinting at the fact that its usage extends significantly beyond the scope simply covered here.

To illustrate, here is a TypeScript example demonstrating memoized computations within a component using useMemo:

const MyComponent: React.FC<Props> = ({data}: Props) => {
  const processedData = useMemo(() => {
    // Heavy computation here
  }, [data]);

  return (

In this instance, the heavy computation only activates when data changes.

On the other hand, useEffect caters to a wider range of use-cases. This includes data fetching, subscriptions, or even manual DOM manipulations. Additionally, it's vital to understand that useEffect runs after all renders unless a dependency array is provided. Contrastingly, useMemo will only re-run the function when its dependency variables change. Recognizing this differentiation is crucial to understand their comparative analysis.

Remember: useMemo specializes in optimizing expensive computations, while useEffect manages the execution of side effects within React components.

The Decision to Memoize All React Components

Contrary to popular belief, memoization does not always enhance performance. Misconceptions of this nature can lead to unneeded complexity and wastage of memory.

Memoization should preferably be employed for components possessing expensive render operations or those that re-render quite often. Otherwise, such optimizations become redundant, creating more inefficiencies than improvements.

Keep in mind: Memoized components stay in memory if they aren't updating to new prop values, and this could lead to memory overhead.

Here's a TypeScript example employing React.memo:

const MyComponent: React.FC<Props> = React.memo((props: Props) => {
  // component code

This specific component will only trigger a re-render when its props change.

Beware of Common Mistakes

Consider this common pitfall:

// Incorrect way
const MyComponentAll: React.FC<Props> = React.memo((props: Props) => {
  // Incorrect as it applies memo to all components, irrespective of their requirements - avoid this overuse of React.memo

In this case, every component in the application is memoized, without accounting for individual requisites. This form of overuse inflates complexity and drains memory resources, thereby inducing more inefficiencies than improvements. Always evaluate a component's re-rendering needs before resorting to memoization.

Wrapping this up, understanding and aptly implementing memoization techniques within React can considerably ramp up application performance. That said, it becomes indispensable to use these practices judiciously, aligning them with the specific needs of your application. Overengineering and imprudent use might initiate unnecessary complexities and an unwarranted drain on resources.

Take a moment to consider the trade-off: does your application of memoization lead to significant performance enhancement? Or does it merely consume excessive memory while introducing redundancy? Striking the perfect balance is the key!

Your turn now - deep dive into these practices and apply them. How effective are you in integrating memoization within your project? Are there instances where the conventional useEffect hook suffices your needs better? Or, do your components require the specialized attention of React.memo or useMemo? Have fun exploring these facets!

Deciding Between React.memo and PureComponent.

Deciding to use React.memo or React.PureComponent in your applications depends heavily on the specifics of your use case. Both these tools are powerful for optimizing React applications, but they each have their best-suited scenarios. Let’s dive into the details.

Understanding PureComponent

PureComponent is a class-based equivalent to React.memo(), and it serves a similar purpose of preventing unnecessary component re-rendering. Its function is to shallowly compare the old and new state or props, and re-render only if this comparison reveals changes. Let’s look at its basic usage within a TypeScript context.

class MyComponent extends React.PureComponent {
    // Here a pure component MyComponent is declared
    render() {
        return <div className='my-component'>{'I am a PureComponent'}</div>; // MyComponent renders a div with a text.

In the code snippet above, MyComponent only re-renders when its state or props change, this can be beneficial for performance. However, PureComponent isn't always the best choice. Its shallow comparison mechanism can miss changes in deeper data structures. If props passed to PureComponent change deeply but not on the surface, it will not trigger a re-render, leading to stale UI. Moreover, PureComponent is restricted to class components, its advantage fades away in functional ones, thanks to the introduction of Hooks.

Effectively using React.memo

React.memo() is a higher-order component that provides similar functionality to PureComponent, but is primarily used with functional components. It also memoizes a component to prevent re-rendering if the props haven't changed.

Here's usage demonstration of React.memo() in a functional component within TypeScript:

const MyComponent: React.FC = React.memo(() => {
    // Here a memoized functional component MyComponent is declared
    return <div className='my-component'>{'I am a memoized component'}</div>; // It renders a div with a text.

Like PureComponent, React.memo() also performs a shallow comparison, which can miss deep prop changes. However, it allows for custom comparison function definition which could handle deep comparison better.

Nevertheless, using React.memo() isn't a win-win. For simple components, the overhead cost of memorization might outweigh the benefits. Indiscriminate use of React.memo() can lead to memory bloat and even more performance issues.

Interview Questions and Scenarios

Question 1: Consider a component that renders a large list of data and does not receive much interaction. Would you use React.memo or PureComponent for optimization?

Question 2: Consider a component that takes in an object prop and only uses a single property of the object. With deep changes that affect only unused properties, how would React.memo and PureComponent behave?

Bad practice alert: Look at the following example – a component receiving a prop that it doesn't use. Even if this prop changes, it doesn't affect our component. Yet, if you use React.memo or PureComponent here, unnecessary re-renders will happen.

const UnoptimizedComponent: React.FC = React.memo(({ unusedProp }) => {
    // This component does not use the prop it receives
    return <div className='unoptimized-component'>{'I am an unoptimized component'}</div>;

Remember, transition to PureComponent or React.memo happens only when you face performance issues. Never default to these constructs immediately when creating new components as they can introduce unnecessary complexity and potential issues to your codebase.

The core philosophy here is that each tool has its beneficial scenario. PureComponent fits well with class components that render large static datasets. React.memo() is better tailored for functional components facing unnecessary re-renders as it allows for customizing the comparison logic. But importantly, either should only be deployed when you observe performance dips. Over-optimization can cause more bugs than it fixes.

Question 3: What approaches can you take to handle deep comparisons in React.memo()?

Question 4: Under what conditions the overhead of memoization outweighs its benefits? Consider this and share your thoughts.

Leveraging useMemo: Enhancing Your Real-life and Interview Performance.

Let's delve into useMemo with some advanced use-cases by scrutinizing multiple TypeScript scenarios that simulate real-world situations. We will focus primarily on useMemo, and you'll find that the concepts we discuss here are transferable to memo.

Understanding Through Code

Let's kick off with an illustrative example of how to use the useMemo function in TypeScript. We'll filter an array of objects based on a search input - a scenario that's commonplace in web applications:

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

type User = { 
  id: number,
  name: string, 

const userList: User[] = [
  { id: 1, name: 'Alex' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' },
  // more users... 

const MemoFilter: React.FC = () => {
  const [search, setSearch] = useState('');

  const filteredUsers = useMemo(() => {
    return userList.filter((user: User) =>;
  }, [search]);

  return (
      <input value={search} onChange={e => setSearch(} />
        { => (
          <li key={}>{}</li>

Applying useMemo ensures that the computationally heavy task of filtering the array only triggers when search changes, thereby enhancing the app's performance.

The Double-Edged Sword: useMemo and memo

Although useMemo and memo have the power to optimize your app's speed, misuse can become counterproductive. Overuse or deployment in inappropriate situations can lead to pitfalls and performance degradation.

The Impact of Misuse

To further highlight this point, consider a situation where the color of a button is determined based on an external state. Intuitively, using useMemo to calculate the color seems to make sense. However, the computation involved is so minimal that the overhead costs of useMemo outweigh any potential benefits. It's paramount to know that while useMemo can be a potent tool for heavy computations, not all calculations warrant memorization.

Handling useMemo in Interviews

We often assign an exaggerated importance to useMemo in coding interviews, speculating its presence in all test scenarios. This misconception unveils a lack of nuanced understanding.

Reflective Questions

To fortify your understanding and interview readiness, consider the following hypothetical questions that could be posed to assess your comprehension of useMemo.

  • When is useMemo appropriately used?
  • Can you identify situations where utilizing useMemo would be detrimental?
  • What changes in the codebase might lead to useMemo becoming a performance bottleneck?

By critically thinking about these questions and applying useMemo more adeptly, you’ll confidently navigate memory management and computational efficiency in React applications.

memo and useMemo: Tackling Overuse and Misuse.

Both memo and useMemo are powerful instruments in the ReactJS collection, instrumental for optimizing your applications by thwarting needless re-computations or re-renderings. Though, akin to all potent tools, misuse and overuse can lead to unintentional consequences or sluggish code.

The Overuse and Misuse

The primary problem is the misconception that memo and useMemo are the ultimate solutions for all performance problems. Utilizing them irrespective of conditions can result in precisely the problem they were meant to remedy: performance degradation. The unnecessary memoization of standard JavaScript computations or React components, which do not consume significant computational power to begin with, may result in an amplified memory footprint. Such incorrect practices underscore the importance of understanding not only how to use these functions correctly but also when to use them. Therefore, always verify the performance impact before and after implementing them, to ensure their use is advantageous.

Consider this TypeScript example which incorrectly applies React.memo to a simple functional component:

import React, { memo } from 'react';

// Misapplication of React.memo 
const MyComponent = memo((props: { title: string }) => (

// Here, React.memo is overkill, potentially consuming more memory than necessary.

In the example above, using React.memo on the function component MyComponent is unnecessary, as the component doesn't involve any heavy computations warranting memoization. Rather than enhancing application performance, it will consume more memory.

Shallow Comparison of Props

Another typical misuse of React.memo and useMemo is overlooking the fact that they perform a shallow comparison of props. Thus, when used wrongly with complex data structures, the result may not match the expected one, necessitating careful consideration of component prop usage.

Consider this incorrect implementation:

import React, { useMemo } from 'react';

const MyComponent = (props: { data: Array<string> }) => {
  const sortedData = useMemo(() =>, []); 
  // This will not behave as expected!

  return (
      { => <li>{item}</li>)}
// useMemo doesn’t save the computation here due to array being a reference type.

In this scenario, although the data could be identical, each time new props are passed to the component, useMemo triggers a sorting computation, as it performs a shallow comparison and arrays are reference types. In these situations, you may need to perform deep comparisons manually or utilize a library such as Lodash for this purpose.

Correct Usage

Having recognized overuse and misuse, how can memo or useMemo be correctly apply? A beneficial approach is conducting a manual check for situations where computations or renderings are noticeably expensive. A typical candidate for this could be a component rendering a massive data list or an operation involving complex computations. Begin by profiling the app performance, thereafter applying memoization if it appears beneficial, and concluding with further profiling to ensure an improvement was indeed achieved.

To illustrate these performance checks in TypeScript with useMemo, consider the following:

import React, { useMemo } from 'react';
import { performance } from 'perf_hooks';

const MyComponent = (props: { data: bigDataStructure }) => {
  const startTime =;
  const processedData = expensiveComputation(; // Without useMemo
  const elapsedTimeNoMemo = - startTime;

  startTime =;
  const memoizedData = useMemo(() => expensiveComputation(, []); // With useMemo
  const elapsedTimeMemo = - startTime;

  console.log('Elapsed time without useMemo:', elapsedTimeNoMemo);
  console.log('Elapsed time with useMemo:', elapsedTimeMemo);

  return (
    // Component that does something with processedData or memoizedData

This snippet exhibits a way of benchmarking the performance of a computation without and with useMemo, helping to make an informed decision about implementing memoization.

In addition, consider the complexity of data being compared. For arrays or object types, a deep comparison function might be necessary.

In conclusion, recognizing useMemo and memo misuse in ReactJS hinges on understanding when these functions should be applied and considering the implications of using them without due diligence. They are potent tools that should be used wisely to truly optimize your JavaScript applications' performance. Will you be altering your current use of memoization in ReactJS?

Recapitulating memo and useMemo: Essential Takeaways and Future Challenges.

The basis of performance optimization in React is built upon the idea of avoiding unnecessary re-renders. Two powerful tools to achieve this are memo() and useMemo().

The memo() function is a higher-order function that's used to wrap React components to prevent unnecessary re-rendering whenever the parent updates, but the props passed to the memoized component remain the same. This judicious use of memo() improves the component's rendering performance dramatically, and prevents the unnecessary use of memory resources. It, however, adds an increased complexity to the system, as it stores a snapshot of the component, increasing the memory footprint.

const MyMemoComponent = React.memo(function MyComponent(props) {
  /* render using props */

The useMemo() hook, on the other hand, is used for creating memoized values rather than components. That is, it only recalculates the memoized value when one of the dependencies has changed. E.g., it can be beneficial when performing expensive calculations.

const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

In terms of complexity, it is vital to consider when and where to use memoization. While they are indeed powerful tools, excessive use of them can lead to “over-optimization” as well as reduced readability of code due to an extra layer of complexity.

These techniques offer a significant ability to improve the performance of your React app, but do not come without challenges: knowing when and where to use memoization can be tricky. You need to assess whether the performance benefits outweigh the added complexity and potential increased memory usage.

React's memo() and useMemo() raise a key question: How can we smartly decide which components or calculations to memoize to balance performance, readability, and memory usage?

React leaves that decision to developers, and the answer changes from one use-case to the other. Perhaps future React versions or community-driven libraries could introduce more intelligent and automated heuristics to address this balancing act.

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