how to stop my timer after it reaches below zero

How come either of these options dont work to stop my timer once it reaches below zero?

var timeLeft = 20

let timer = () => {
  setInterval(() => {
    timeLeft--
    timeLimitEl.textContent = timeLeft
    console.log(timeLeft)
    if (timeLeft < 0) {
      // option 1 - did not work
      // clearInterval(timer)
      // option 2 - did not work
      // return
    }
  }, 1000)
}

setInterval returns an id of the interval. You have to call clearInterval with that id

var timeLeft = 5
let handle

let timer = () => {
  handle = setInterval(() => {
    timeLeft--
    //timeLimitEl.textContent = timeLeft
    console.log(timeLeft)
    if (timeLeft < 0) {
      clearInterval(handle)
    }
  }, 1000)
}

timer()