---
example:
  title: Timer Button
  instructions: >
    Create a function called `updateTimer()` containing the timer updating logic.
    Then use `setInterval(updateTimer, 1000)` to update the timer every second
    when the user clicks **Start**.
  hint: >
    Store the timer id in the button node and use `clearInterval()` to stop the timer
    when the user clicks **Stop**.
  initial:
    js: |+2
      const timerBtn = document.getElementById('timer-button')
      
      timerBtn.addEventListener('click', ev => {
        const btn = ev.currentTarget
        const action = btn.value
        console.log(action)
      
        if (action === 'start') {
          btn.value = 'stop'
          btn.textContent = 'Stop Timer'
        } else if (action === 'stop') {
          btn.value = 'start'
          btn.textContent = 'Start Timer'
        }
      })
  solution:
    js: |+2
      const timerBtn = document.getElementById('timer-button')

      const updateTimer = () => {
        const then = timerBtn.startTime
        const now = new Date()
        let seconds = Math.floor((now - then) / 1000)
        
        let minutes = 0
        if (seconds >= 60) {
          minutes = Math.floor(seconds / 60)
          seconds = seconds - minutes * 60
        }
        
        minutes = (minutes < 10 ? '0' : '') + minutes
        seconds = (seconds < 10 ? '0' : '') + seconds
        
        document.getElementById('timer-minutes').textContent = minutes
        document.getElementById('timer-seconds').textContent = seconds
      }
      
      timerBtn.addEventListener('click', ev => {
        const btn = ev.currentTarget
        const action = btn.value
      
        if (action === 'start') {
          btn.value = 'stop'
          btn.textContent = 'Stop Timer'
          btn.startTime = new Date()
          btn.timerId = setInterval(updateTimer, 1000)
        } else if (action === 'stop') {
          btn.value = 'start'
          btn.textContent = 'Start Timer'
          clearInterval(btn.timerId)
        }
      })
    css: ~
output: js4shiny::html_document_plain

---

<div class="controls">
  <button id="timer-button" value="start">Start Timer</button>
</div>

<div id="timer">
  <span id="timer-minutes">00</span>:<span id="timer-seconds">00</span>
</div>