Edit in JSFiddle

var btn = document.getElementById("btn");
var result = document.getElementById("result");
btn.onclick = function(){
    var a;
    if ('http://giftwrapped.com/picture.jpg'.match('.gif'))     {
     a = 'a gif file'
    } else {
     a = 'not a gif file'
    }
    result.innerHTML = a; 
}

<p>点击查看结果：</p>
<button id="btn">查看结果</button>
<p>结果：</p>
<div id="result"></div>

p {
    font-size: 14px;
    height 24px;
    line-height: 24px;
}