Добавить значение в textbox

const alph = "abcdefghijklmnopqrstuvwxyz"; 
const textarea = document.querySelector('textarea'); 
const button = document.querySelector('button'); 
 
 
button.addEventListener('click', function() { 
 let result =  generator.next(); 
  textarea.textContent +=  result.value||''; 
}) 
 
const generator = addChar(alph); 
 
function* addChar(str) { 
  for (let i = 0; i <str.length; i++) { 
    yield str[i]; 
  } 
  return ''; 
}

<textarea></textarea> 
<button>click me </button>

const letters = 'abcdefghijklmnopqrstuvwxyz' 
const textInput = document.querySelector('input[type="text"]') 
document.querySelector('input[type="button"]').addEventListener('click', 
  e => { 
    let currentValue = textInput.value 
    if (currentValue.length >= letters.length) return 
 
    textInput.value = currentValue + letters[currentValue.length] 
  })

<input style="width:100%" type="text" /> 
<input type="button" value="+" />