Ajax接收數(shù)據(jù)時(shí)，為什么給span的外面寫(xiě)上【】？

window.onload = function() {

var oBtn = document.getElementById('btn');


oBtn.onclick = function() {
    
    var xhr = null;
    try {
        xhr = new XMLHttpRequest();
    } catch (e) {
        xhr = new ActiveXObject('Microsoft.XMLHTTP');
    }
    
    xhr.open('get','getNews.php',true);
    xhr.send();
    
    xhr.onreadystatechange = function() {
        
        if ( xhr.readyState == 4 ) {
            if ( xhr.status == 200 ) {
                //alert( xhr.responseText );
                var data = JSON.parse( xhr.responseText );
                
                var oUl = document.getElementById('ul1');
                var html = '';
                for (var i=0; i<data.length; i++) {
                    html += '<li><a href="">'+data[i].title+'</a> [<span>'+data[i].date+'</span>]</li>';//問(wèn)題點(diǎn)：[<span>'+data[i].date+'</span>]為什么給span的外面寫(xiě)上[]?
                }
                oUl.innerHTML = html;
            } else {
                alert('出錯(cuò)了,Err：' + xhr.status);
            }
        }
        
    }
    
}

}
</script>