How to get selected(user-highlighted) text in contenteditable element and replace it?

<div class='a'>I am pritom</div>

var parent = $(".a");
parent.find("span.2406201317592000").remove();
var text = common__GetSelectionHtml();
if(parent.find("span.2406201317592000").length) { /* means text selected in this section */
    console.log(text);
}

function getSelectionHtml() {
    var html = "";
    if (typeof window.getSelection != "undefined") {
        var sel = window.getSelection();
        if (sel.rangeCount) {
            var container = document.createElement("div");
            for (var i = 0, len = sel.rangeCount; i < len; ++i) {
                container.appendChild(sel.getRangeAt(i).cloneContents());

                var newSpan = document.createElement("span");
                var range = sel.getRangeAt(i);
                newSpan.setAttribute('class','span_2406201317592000');
                $(newSpan).html("");
                //range.collapse(false);
                range.insertNode(newSpan);
            }
            html = container.innerHTML;
        }
    } else if (typeof document.selection != "undefined") {
        if (document.selection.type == "Text") {
            html = document.selection.createRange().htmlText;
        }
    }
    console.log(html);
}

function replaceSelectionWithHtml(html) {
    var range, html;
    if (window.getSelection && window.getSelection().getRangeAt) {
        range = window.getSelection().getRangeAt(0);
        range.deleteContents();
        var div = document.createElement("div");
        div.innerHTML = html;
        var frag = document.createDocumentFragment(), child;
        while ( (child = div.firstChild) ) {
            frag.appendChild(child);
        }
        range.insertNode(frag);
    } else if (document.selection && document.selection.createRange) {
        range = document.selection.createRange();
        html = (node.nodeType == 3) ? node.data : node.outerHTML;
        range.pasteHTML(html);
    }
}

jQuery.fn.selectText = function(){
   var doc = document;
   var element = this[0];
   console.log(this, element);
   if (doc.body.createTextRange) {
       var range = document.body.createTextRange();
       range.moveToElementText(element);
       range.select();
   } else if (window.getSelection) {
       var selection = window.getSelection();        
       var range = document.createRange();
       range.selectNodeContents(element);
       selection.removeAllRanges();
       selection.addRange(range);
   }
};

$("button").click(function() {
    $("#editable").selectText();
});