Get Selected text from editableContent DIV?

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Javascript

HTML5

AoA,
How can I get the selected text, its begining and ending cursor position, like I did in the following code(which works on textArea)

var tempArea = document.getElementById('MainText').innerHTML;

    var textComponent = document.getElementById(textbox);
    var selectedText;
    var allText = textComponent.value;
    // IE version
    if (document.selection != undefined) {
      //I dont care about IE

    }
        // Mozilla version
    else if (textComponent.selectionStart != undefined) {
        var startPos = textComponent.selectionStart;
        var endPos = textComponent.selectionEnd;
        selectedText = textComponent.value.substring(startPos, endPos);
        if (selectedText.search(arguments[1]) == -1 && selectedText.search(arguments[2]) == -1)
             allText = allText.substring(0, startPos) + arguments[1] + selectedText + arguments[2] + allText.substring(endPos,allText.length);
    }

    document.getElementById(textbox).value = allText;

How can I acheive same thing with editableContent DIV?