<script type="text/javascript">
if(variable){
alert('rain man');
}
</script>

<script type="text/javascript">
if( typeof variable == 'undefined' ){
alert('rain man');
}
</script>

<script type="text/javascript">
var two = {};
if(two.b){
alert('rain man');
}
if( window.addEventListener ){
alert('This is not IE!');
}
</script>

<script type="text/javascript">
var obj = {};
obj.property.number = 2; //這時會出現 'obj.property is undefined' 的錯誤

/**
* 下面這種情況雖然沒有出現語法錯誤,但實際上已經出現了錯誤.
* 屬性是復合型變量所特有的,而obj.property是數值型變量,不能含有屬性.
*/
var obj = {};
obj.property = 2 ;
obj.property.number = 3;
</script>