JavaScript获取文本框内选中文本的方法

<script type="text/javascript">
function getFieldSelection(select_field)
{
    word='';
    if (document.selection) {
        var sel = document.selection.createRange();
        if (sel.text.length > 0) {
            word = sel.text;
        }
    }
    else if (select_field.selectionStart || select_field.selectionStart == '0') {
        var startP = select_field.selectionStart;
        var endP = select_field.selectionEnd;
        if (startP != endP) {
            word = select_field.value.substring(startP, endP);
        }
    }
    return word;
}
</script>
<textarea id="a" rows="3" cols="20">选择我,然后点击狼蚁网站SEO优化的按钮</textarea><br />
<button onclick="alert(getFieldSelection(document.getElementById('a')))">button_click</button>
<br />
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