Memo & useCallback

Best performance and optimization training

Before optimization

• Making enviroment with the issue of all components re-rendering

• Understanding how react works and re-evaluation : re-running everything again in react except states declaring.

• Difference between re-run/re-evaluate & re-render

  • re-evaluate should work every time and compare
  • re-render works on change only with calling react-dom
  • All react components will re-rerun (not re-render) everytime if we didn't use any optimization

Optimization methods

It's a trial to make re-run/re-evaluate works like re-render on changes only

1. React.memo()

• it saves the component states and props to get checked first before the re-run

• It caches the props and states of the components so using it alot in useless way will make it caching a lot so it will be heavy on runtime.

   

   import React from "react"

   const Component=(props)=>{
    //some javascript
    return (
        //some jsx
    )
   }
   export default React.memo(Component)

1. useMemo()

• difference between reference types in Js
  

   

   let name = "adel"
   let name2 = "adel"
  
   //name === name2 --> true
  
   let obj = {name:"adel"}
   let obj2 = {name:"adel"}
  
   //obj === obj2 --> false
   //objects on different refrences values
  
   let obj3 = obj
   //passing by refrence
  
   //obj === obj3 --> true
  

  • Primitive type
  • Refrence type : Object, array & function

     

• useMemo is used with arrays, objects & with functions that returns something to fix the problem in re-decleration with every re-reun
  

   const obj = useMemo(()=>{ return name: "adel" }, [])
  

• useMemo prevents object re-declaration with new refrence on re-run

1. useCallback()

• useCallback() prevents function (with no return) from re-declaration with new refrence on re-rerun

   

   //this won't work it will be re-declared with new refrence every re-run

   const addAge = () => {
     //function logic with no return
   }

   //solution

   const addAge = useCallback(() => {
     //function logic
   }, [])

   return <Component addAge={addAge} />

• useCallback caches the logic itself not a return value like useMemo