JSFiddle - React, Tailwind, and code Playground

HTML

<textarea rows="2" cols="20" id="pPost">mardagz
</textarea> 
<input type="button" id="bbbold" value="clcikMe" />

JavaScript

function getSelectedText() {
    var len =$("#pPost").val().length;
    var start = $("#pPost")[0].selectionStart;
    var end = $("#pPost")[0].selectionEnd;
    var sel = $("#pPost").val().substring(start, end);
    return sel;
}


function bbrep(start, end) {
    var tmpVal = getSelectedText();
    $("#pPost").val($("#pPost").val().replace(tmpVal, start + tmpVal + end));
}

$("#bbbold").click(function() {
    bbrep("[b]", "[/b]");
    return false;
})