get start and end point from document.createrange() method javascript

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AoA,
I have found the following code from somewhere...I have changed the mozilla version and its working fine... but i am unable to get the start and end position of selected text from createrange() method


//JavaScript.js

function ShowSelection() {
    var textComponent = document.getElementById('MainTextArea');
    var selectedText;
    var allText = document.getElementById('MainTextArea').value;
    // IE version
    if (document.selection != undefined) {
        textComponent.focus();
        var sel = document.selection.createRange();
   
        selectedText = sel.text;

        var slen = sel.moveStart('character', -oField.value.length);
        var elen = sel.text.length;
        allText.value = allText.value.substring(0, slen) + "" + selectedText;

    }
        // Mozilla version
    else if (textComponent.selectionStart != undefined) {
        var startPos = textComponent.selectionStart;
        var endPos = textComponent.selectionEnd;
        selectedText = textComponent.value.substring(startPos, endPos);

        allText = allText.substring(0, startPos) + "" + selectedText + "" + allText.substring(endPos,allText.length);
    }

    document.getElementById('MainTextArea').value = allText;
}

although mozilla version is running fine..
is there any way to get start and end position from createrange() method?