Edit in JSFiddle

var btn = document.getElementById("btn");
var result = document.getElementById("result");
btn.onclick = function(){
	function showCase(value) {
    var a;
    switch(value) {
    case 'A':
        a = 'Case A';
        break;
    case 'B':
        a = 'Case B';
        break;
    case undefined:
        a = 'undefined';
        break;
    default:
        a = 'Do not know!';
    }
    return a;
}
  result.innerHTML = showCase( String('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;
}