JS custom functions with default values

At first, I think it may be the same as in php or python.
Js Code Copy codeThe Code is as follows: function test (a, B = "er "){
Alert (a + B );
}

However, after the definition is complete, an error is reported: the object is missing. It is found that javascript cannot be defined in this way. The arguments is a real parameter array. See the following example:
Js CodeCopy codeThe Code is as follows: <script type = "text/javascript">
Function Test (){
Var B = arguments [1]? Arguments [1]: 50;
Return a + ':' + B;
}
Alert (Test (5 ));
Alert (Test (5, 9 ));
</Script>

After the test, it turned out to be. It seems that everything has its own laws and cannot be taken for granted. Record it here to avoid making mistakes later.