JavaScript code considerations

1 The line at the end of the code to add good. Reason <<javascript Advanced Programming Third Edition, page 21, line three >>

2 initialization variables should be added with default values
Because values that are not declared and declared as initialized when using typeof return undefined, if the rules are initialized to get undefined, you know that there is no declaration
VAR message;
Alert (typeof message)//undefined
Alert (typeof age)//undefined age no statement

3 If a defined variable is used to hold an object, it is best to initialize it to null, so as long as you check if NULL, you know if you have saved a reference to an object.

4 undefined = = NULL//true

5 Nan = = Nan//false
IsNaN (object), parsing method, JS Advanced Program Design Third edition 30 page fifth line

JavaScript code considerations