1. logic operation : '&&' and '||' .For this two logic operations,its' results are inconclusive.the difference between this two logic operations and others is that they don't change the data type of the operands and also don't change the data type of the operation result. In the expression , this operator(&& and ||) will understand the type of the operands as the bool type ,so they can execute bool operation.you can have a look below.
1 <html> 2 <head> 3 <meta http-equiv="content-type" charset="utf-8"/> 4 <script type="text/javascript"> 5 var str = "hello"; 6 var obj = {}; 7 alert(str || obj);//str as string type ,obj as an object,return str("hello") 8 alert(str && obj);//return obj 9 alert(undefined || 123);//undefined as false,and return 123 10 </script> 11 </head> 12 <body> 13 </body> 14 </html>
2. comparison operation. For comparing two value type data,generally just compare their values.For comparing a value type data and a reference type data,first switch the reference type data to the same type as the value type data,then compare them.For comparing two reference type data,it's unmeaning and always return false.like below:
1 <html> 2 <head> 3 <meta http-equiv="content-type" charset="utf-8"/> 4 <script type="text/javascript"> 5 var o1 = {}; 6 var o2 = {}; 7 var str = "123"; 8 var num = 1; 9 var b0 = false; 10 var b1 = true; 11 var ref = new String(); 12 13 alert(b1 < num);//return false; 14 alert(b1 <= num);//return true; 15 alert(b1 > b0);//return true; 16 17 alert(num > ref);//return true; 18 19 alert(o1 > o1 || o1 < o2 || o1 == o2);//always return false; 20 </script> 21 </head> 22 <body> 23 </body> 24 </html>