如何判断一个变量是否为空？

Question

没问题呀
<script>
var x;
if(x==null){alert("null");}else{alert("true");}
x=true;
if(x==null){alert("null");}else{alert("true");}
</script>

Answer 1

try
if(x){alert("true");}else{alert("null");}<script>
var x;
if(x){alert("true");}else{alert("null");}
x=true;
if(x){alert("true");}else{alert("null");}
</script>

Answer 2

if(x) alert("yes!");
else alert("No!");

Answer 3

if(x) alert("yes!");
else alert("No!");

Answer 4

if(x) alert("yes!");
else alert("No!");

Answer 5

<script>
if(typeof(x)!="undefined") alert("x 存在.x="+x);else alert("x 不存在");
</script>