How to execute or stop a function on toggle switch using plain javascript?

/* A let variable is declared, not a const, to change the content. */
let checkBox = document.getElementById('checkbox');
let input = document.getElementById('input');

function transRussian(ch){
  ch = ch.replace(/l/g, "л");
  ch = ch.replace(/p/g, "п");
  return ch;
}

function translateBlock(){
  var ch = input.value;
  input.value = transRussian(ch);
}

/* When the checkbox is clicked, the control variable is set to true. */
input.addEventListener('keyup', () => {
  if(checkBox.checked)
    translateBlock();
});

/* This method is triggered when data is entered in the <textarea> element. */
checkBox.addEventListener('change', () => {
  if(checkBox.checked)
    translateBlock();
  else
    console.log("transliteration is turned off")
});

textarea{
  display: block;
}

<span>Translate</span>
<input type="checkbox" id="checkbox"/>

<textarea id="input" name="w3review" rows="4" cols="50">
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</textarea>