Typescript version of Dan Abramov's useInterval requiring non-null value for delay

use-interval.ts

import * as React from 'react'

const { useState, useEffect, useRef } = React

type IntervalFunction = () => ( unknown | void )

function useInterval( callback: IntervalFunction, delay: number ) {

  const savedCallback = useRef<IntervalFunction| null>( null )

  // Remember the latest callback.
  useEffect( () => {
    savedCallback.current = callback
  } )

  // Set up the interval.
  useEffect( () => {
    function tick() {
      if ( savedCallback.current !== null ) {
        savedCallback.current()
      }
    }
    const id = setInterval( tick, delay )
    return () => clearInterval( id )

  }, [ delay ] )
}

In both, 'useState' is declared but its value is never read. 😄 . (Also the case in Dan's original post)