JavaScript - Como seleccionar una parte del texto de un textare

// Gets the selection in a textfield

function getSelection(myField)

{

    //IE support

    if (document.selection)

    {

        myField.focus();

        sel = document.selection.createRange();

        return sel.text;

    }

    //MOZILLA/NETSCAPE support

    else if (myField.selectionStart || myField.selectionStart == "0")

    {

        var startPos = myField.selectionStart;

        var endPos = myField.selectionEnd;

        return myField.value.substr(startPos, endPos - startPos);

    } else {

        return "";

    }

}