📖 History of "Stop unnecessary re-rendering component in React !!"

once upon time...

💎 Generation of class component

🚩 Pure component()

Compare new and old props/state, if there is difference between them, component renders

compare?? but how to compare them??

<< the case of rendering in React >>

1. state changes
2. parent component renders
3. props changes
4. shouldcomponentUpdate function returns true (I'll explain about it later)
5. forceUpdate

for numbers 1 and 2, React decides whether to render through shallow compare

What is shallow compare?

At first, we need to get what is reference
▼ from this website

1. pass by reference (shallow copy)
   If you pour coffee in copied cup, original cup is also filled with it (because both datas are in same memory allocation space)

2. pass by value (deep copy)
   If you pour coffee in copied cup, original cup is still empty

in Javascript, primitive data type (String, Number, Bigint, Boolean, Undefined, Symbol) is pass by value, and Object, Array is pass by reference

honestly comparing with primitive data type is not so difficult, but we need to care about comparing with Object

the case of object reference is the same

import shallowCompare from 'react-addons-shallow-compare';

const a = { country: "poland", country2: "japan" }
const b = a

console.log(shallowEqual(a, b))
// true

the case of object reference is different

1. not nested object

import shallowCompare from 'react-addons-shallow-compare';

const a = { country: "poland", country2: "japan" }
const b = { country: "poland", country2: "japan" }

console.log(shallowEqual(a, b))
// true

1. nested object

import shallowCompare from 'react-addons-shallow-compare';

const a = {
  country: "poland",
  coountry2: {
    city1: "tokyo",
    city2: "osaka"
  }
}

const b = {
  country: "poland", // country is primitive type, scalar data is the same -> true
  country2: { // country2 is object, so reference is different -> false
    city1: "tokyo",
    city2: "osaka"
  }
}

console.log(shallowEqual(a, b))
// ⭐ false

🚩 shouldComponentUpdate()

👦 so it is ok all components are pure component, isn't it?
👩‍💻 no, because cost of comparing old and new state/props is high
👦 what should I do then?
👩‍💻 just decide comparing condition by yourself via "shouldComponentUpdate()"

actually PureComponent is like component which is implemented by someone(would be someone in facebook company) through shouldComponentUpdate()

// something like that
class PureComponent extends React.Component {
    shouldComponentUpdate(nextProps, nextState) {
        return !(shallowEqual(this.props, nextProps) && shallowEqual(this.state, nextState));
    }
    …
}

---

💎 Functional Component generation

2022 we are in this generation

🚩 React.memo

it is like PureComponent() + shouldComponentUpdate()

// if new props changes, this component will be rendered
const Button = React.memo(props => {
    return <div>{props.value}</div>
})

// if you put second argument, it is like shouldComponentUpdate()
const Button = React.memo(
    props => {
        return <div>{props.value}</div>
    },
    (nextProps, prevProps) => {
        return nextProps.value === prevProps.value
    }
)

🚩 useMemo

👦 what is useMemo? it is the same of React.memo?
👩‍💻 no, similar though. React.memo is added by React version16.6, and then React hook is added by version 16.8, useMemo as well.
👦 aha
👩‍💻 useMemo renders only when props changes because it remembers calculation result

// when only "products props" changes, this component renders
const Component: React.FC = ({ products }) => {
    const soldoutProducts = React.useMemo(() => products.filter(x => x.isSoldout === true), [products])
}

🚩 useCallback

When parent component passes props of function to child component, new function (actually function is just one of object) is made.
Because of it child component recognize this new function is different from old one, then re-renders sadly.

↓ conversation between child/parent component

👨 Parent「re-renderiiingggg!! And now I recreated function that I have !!」
👼 Child「Mom!! Give me your function as props!!」
👨 Parent「ok I give you my function!」
👼 Child「well now I need to confirm this object’s memory address is the same as object that I got before …. Hmmm different address, I also re-rendering!!!」

to prevent this unnecessary re-rendering, should use useCallback

Comments

[Tris]

and Object, Array is pass by reference
I dont think it's actually true. In JS, we only have pass-by-value. Pass-by-reference means we will pass the ADDRESS of VARIABLE. But in JS, we pass the object which variable is referring. So it's not pass-by-reference

[Jorge Ramón]

Everything in JavaScript is pass-by-reference so if your function modifies an object or array then the change applies to the original object.

The case of the "pass-by-value" in primitive date types it's because primitive data types are immutable so yeah, you are still passing the reference to a function but it doesn't matter cause its immutability

[Tris]

Please take a look
stackoverflow.com/questions/131044...

[Jorge Ramón]

Thanks, exactly what I said but remember primitives are immutable so that why they act as "pass-by-value"

[Tris]

Oh, let me confirm again. In JS, we only "pass-by-value"

[Jorge Ramón]

Okay, I got your point.

Technically, JS works "pass-by-value" since you pass a copy of a reference. But if you modify that copy of the reference then it modifies the original object so that's basically "pass-by-reference" xd

Same as Primitive Data Types Immutability is basically the same as pass-by-value.

So it depends, you can see the glass half empty or half full

[Tris]

Agree with you

[Andrews Kangah]

I think there's problem with React.memo example. Change determination function should be like the following instead...
return nextProps.value !== prevProps.value

Not like the following...
return nextProps.value === prevProps.value

Because it should re-render only when old value not equal to new value

[Danae]

Very informative! Would be nice to have an example for useCallback too though! It is not very clear to me :/

[Rainson12]

+1 for the example of the useCallback. Would appreciate it.

[AtomEistee]

Amazing explanations, but my small problem is that i quite dont understand when exactly do i need one of this things, i guess i will understand it with time cuz i think there is no universal rule for each website. Thanks for this article.

[Tao Wen]

re-rendering could also be caused by react context, it can skip your parent and directly re-render some child