<button id="btn1">Click Me!</button>

const btn = document.getElementById('btn1')

// keep track of how many time we
// clicked on the button.
let count = 0

const onClick = () => {
  count += 1 // increment the counter
  
  // "innerHTML" is an API
  btn.innerHTML = `click count: ${count}`
}

btn.addEventListener('click', onClick)
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