鼠标选中文字的兼容

function getSelectedText() {
    var txt = ""; 
    if (window.getSelection) { 
        txt = window.getSelection(); 
    } else if (window.document.getSelection) { 
        txt = window.document.getSelection(); 
    } else if (window.document.selection) {
        txt = window.document.selection.createRange().text; 
    } 
        return txt; 
    }