firstly please review a code sample below .
<html>
<head>
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script type="text/javascript">
/**/
$(function(){
var baseObj=function(){
this.firstname="joe";
this.lastname="wang";
};
var childObj=function(){
this.firstname="wq";
};
var b= new baseObj();
var c = new childObj();
baseObj.call(c);
alert(c.firstname);
});
</script>
</head>
<body>
</body>
</html>
the output is "joe" instead of the "wq".
so, found the mystery the call ?
Another case need to be mentioned here , the firstArg of call could be null.
As the MDN explained.
thisArg
The value of this provided for the call to fun. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed.
Simply put it by a little code.
<script>'use strict'function fun(){
alert(this);}
fun.call(null);</script>
In the strict mode ,this is null. But in the not strict mode . this is window or global object.
see also:
http://stackoverflow.com/questions/4166616/understanding-the-difference-between-object-create-and-new-somefunction-in-j