for/in 语句循环遍历对象的属性，注意： for...in 循环中的代码块将针对每个属性执行一次

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>JavaScript</title>
</head>

<body>
<p>点击下面的按钮，循环遍历对象 "person" 的属性</p>
<button onClick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
function myFunction(){
var x="";
var txt="";
var person={fname:"Bill",lname:"Gates",age:56};
for(x in person){
    txt=txt+person[x]+" ";
}
document.getElementById("demo").innerHTML=txt;
}
</script>
</body>
</html>