gatsby-plugin-transitions/src/transitions/hooks/usePrev.js

Gatsby-plugin for animated page-transitions.

import { useEffect, useRef } from 'react'

export default function usePrevious (value) {
  // The ref object is a generic container whose current property is mutable ...
  // ... and can hold any value, similar to an instance property on a class
  const ref = useRef()

  // Store current value in ref
  useEffect(() => {
    ref.current = value
  }, [value]) // Only re-run if value changes

  // Return previous value (happens before update in useEffect above)
  return ref.current
}