Here's how you can do it, in all major browsers. I've also got a jQuery plug-in that includes this functionality. With that, the code would be

$(your_textarea_id).replaceSelectedText(NEW TEXT);

Here's a full stand-alone solution:

function getInputSelection(el) {

    var start = 0, end = 0, normalizedValue, range,

        textInputRange, len, endRange;



    if (typeof el.selectionStart == number && typeof el.selectionEnd == number) {

        start = el.selectionStart;

        end = el.selectionEnd;

    } else {

        range = document.selection.createRange();



        if (range && range.parentElement() == el) {

            len = el.value.length;

            normalizedValue = el.value.replace(/rn/g, n);



            // Create a working TextRange that lives only in the input

            textInputRange = el.createTextRange();

            textInputRange.moveToBookmark(range.getBookmark());



            // Check if the start and end of the selection are at the very end

            // of the input, since moveStart/moveEnd doesn't return what we want

            // in those cases

            endRange = el.createTextRange();

            endRange.collapse(false);



            if (textInputRange.compareEndPoints(StartToEnd, endRange) > -1) {

                start = end = len;

            } else {

                start = -textInputRange.moveStart(character, -len);

                start += normalizedValue.slice(0, start).split(n).length - 1;



                if (textInputRange.compareEndPoints(EndToEnd, endRange) > -1) {

                    end = len;

                } else {

                    end = -textInputRange.moveEnd(character, -len);

                    end += normalizedValue.slice(0, end).split(n).length - 1;

                }

            }

        }

    }



    return {

        start: start,

        end: end

    };

}



function replaceSelectedText(el, text) {

    var sel = getInputSelection(el), val = el.value;

    el.value = val.slice(0, sel.start) + text + val.slice(sel.end);

}



var el = document.getElementById(your_textarea);

replaceSelectedText(el, [NEW TEXT]);