CODE EXAMPLE FOR JAVASCRIPT

useMemo example:

import React, { useEffect, useMemo } from "react"
import { massiveArray } from "./massiveArray"

export const filterAndMapMassiveArrayToNumber = number => {
  // There are much better ways and algorithms to do this sorting,
  // this is only for illustrating the purpose of useMemo.
  return massiveArray
    .map(item => ({
      ...item,
      value: item.value * myNumber,
    }))
    .filter(item => item.value > 10)
}

export default function UseMemoWithExpensiveFunctionExample({ myNumber = 1 }) {
  const finishedArray = useMemo(
    () => filterAndMapMassiveArrayToNumber(myNumber),
    [myNumber]
  )

  return (
    <ul>
      {finishedArray.map(item => (
        <li key={item.id}>{item.value}</li>
      ))}
    </ul>
  )
}