How To Wrap / Surround Highlighted Text With An Element

function wrapSelectedText() {       
    var selection= window.getSelection().getRangeAt(0);
    var selectedText = selection.extractContents();
    var span= document.createElement("span");
    span.style.backgroundColor = "yellow";
    span.appendChild(selectedText);
    selection.insertNode(span);
}

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<input type="button" onclick="wrapSelectedText();" value="Highlight" />

JS Fiddle.

If the selection is completely contained within a single text node, you can do this using the surroundContents() method of the range you obtain from the selection. However, this is very brittle: it does not work if the selection cannot logically be surrounded in a single element (generally, if the range crosses node boundaries, although this is not the precise definition). To do this in the general case, you need a more complicated approach.

Also, DOM Range and window.getSelection() are not supported in IE < 9. You'll need another approach again for those browsers. You can use a library such as my own Rangy to normalize browser behaviour and you may find the class applier module useful for this question.

Simple surroundContents() example jsFiddle: http://jsfiddle.net/VRcvn/

Code:

function surroundSelection(element) {
    if (window.getSelection) {
        var sel = window.getSelection();
        if (sel.rangeCount) {
            var range = sel.getRangeAt(0).cloneRange();
            range.surroundContents(element);
            sel.removeAllRanges();
            sel.addRange(range);
        }
    }
}