index.html

<style>
  button {
    font-size: 1.5rem;
  }
  
  span {
    font-weight: bold;
    margin-left: 1rem;
  }
</style>

<button>+</button>
<span>0</button>

<script>
  document.querySelector('button').addEventListener('click', () => {
    const span = document.querySelector('span')
    const i = parseInt(span.innerText, 10)
    span.innerText = String(i+1)
  })
</script>