JAVASCRIPT

change style selected text js

function changeFont(font) {
  var sel = window.getSelection(); // Gets selection
  if (sel.rangeCount) {
    // Creates a new element, and insert the selected text with the chosen font inside
    var e = document.createElement('span');
    e.style = 'font-family:' + font.value + ';'; 
    e.innerHTML = sel.toString();

    // https://developer.mozilla.org/en-US/docs/Web/API/Selection/getRangeAt
    var range = sel.getRangeAt(0);
    range.deleteContents(); // Deletes selected text…
    range.insertNode(e); // … and inserts the new element at its place
  }
}

change style selected text js

Highlight text and change font <br>
<select id="select_font" onchange="changeFont(this);">
            <option value="Arial">Arial</option>
            <option value="Sans Serif" selected>Sans Serif</option>
            <option value="Comic Sans MS">Comic Sans MS</option>
            <option value="Times New Roman">Times New Roman</option>
            <option value="Courier New">Courier New</option>
            <option value="Verdana">Verdana</option>
            <option value="Trebuchet MS">Trebuchet MS</option>
            <option value="Arial Black">Arial Black</option>
            <option value="Impact">Impact</option>
            <option value="Bookman">Bookman</option>
            <option value="Garamond">Garamond</option>
            <option value="Palatino">Palatino</option>
            <option value="Georgia">Georgia</option>
        </select>
<div contenteditable="true" id="note_header" style="width:200px; height:200px; border: 1px solid #ccc">
  Some Content
</div>

change style selected text js

function changeFont(font) {
  document.getElementById("note_header").style.fontFamily = font.value;
}