This is one your are looking for i believe:
text = window.getSelection().toString();
Hide selected/highlighted text javascript
You have to get parent of Element from DOM:
function getSelectionParentElement() {
    var parentEl = null, sel;
    if (window.getSelection) {
        sel = window.getSelection();
        if (sel.rangeCount) {
            parentEl = sel.getRangeAt(0).commonAncestorContainer;
            if (parentEl.nodeType != 1) {
                parentEl = parentEl.parentNode;
            }
        }
    } else if ( (sel = document.selection) && sel.type != "Control") {
        parentEl = sel.createRange().parentElement();
    }
    return parentEl;
}
Update
Fixed demo to hide text we have to find startOffset
function getStartOffset() {
    var sel = document.selection, range, rect;
    var x = 0, y = 0;
    if (sel) {
        if (sel.type != "Control") {
            range = sel.createRange();
            range.collapse(true);
        }
    } else if (window.getSelection) {
        sel = window.getSelection();
        if (sel.rangeCount) {
            range = sel.getRangeAt(0).cloneRange();
            if (range.getClientRects) {
                range.collapse(true);
            }
        }
    }
    return range.startOffset;
}