[Change Button Text Depending on Another Field] #js

index.html

<form>
	<input class="kb-field-2">
	<button class="kb-forms-submit">A</button>
</form>

<script>
const field = document.getElementsByClassName('kb-field-2')[0];
field.addEventListener("keyup", () => {
    let btn = document.getElementsByClassName('kb-forms-submit')[0];
    if( field.value === "" ) {
     btn.textContent = "A";
    }else{ 
      btn.textContent = "B";
    }
});
</script>