html textarea along with cursor position set
document.getElementById('bold').addEventListener('click', boldText);
function boldText() {
var txtarea = document.getElementById("editor_area");
var start = txtarea.selectionStart;
var end = txtarea.selectionEnd;
var sel = txtarea.value.substring(start, end);
var finText = txtarea.value.substring(0, start) + '[b]' + sel + '[/b]' + txtarea.value.substring(end);
txtarea.value = finText;
txtarea.focus();
txtarea.selectionEnd= end + 7;
}