Clickable tristate checkbox

There is HTMLButtonElement.indeterminate, but you cannot set it normally on click.

This one is quite simple. You need two things.

• JavaScript onclick handler
• Place to hold the state

A simple implementation for this, is to store on Element.target.value, however, it can only be strings (otherwise, it will be coerced to string).

function tristateHandler(e) {
  const states = ['true', 'null', 'false']

  const i = states.indexOf(e.target.value) + 1
  e.target.value = i < states.length ? states[i] : states[0]
  switch(e.target.value) {
    case states[0]:
      e.target.checked = true
      break
    case states[1]:
      e.target.indeterminate = true
      break
    default:
      e.target.checked = false
  }

  // Sadly, e.target.value is coerced to string
  console.log(typeof e.target.value)
}

document.querySelector('input[type=checkbox]').onclick = tristateHandler