Literal variable identifier

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            literal, immutable values are some 
            literals are directly used 
            
            variables can be saved literals, and values of the variables can be changed to 
            a variable we use more convenient, so development is going to save a literal by variables 
            without using a literal 
            
            declaration of variables 
            in js use var keyword to declare a variable 
            declaration and assignment simultaneously 
            
            identifiers 
            in JS and all that are named by our independent identifiers can be called
            For example, the variable name attribute name function names 
            naming identifier rules:
            1. The identifier may contain letters, numbers, _, $ 
            2 identifier can not begin with a number 
            3. ES identifier can not be reserved words or keywords 
            4. camelCase identifier generally used method 
            first letter in lower case, each capital letter initials, the rest in lower case letters 
            
            while saving is actually employed JS identifier identifying the underlying Unicode encoding 
            so in theory, all matter contained in utf-8 can be used as an identifier 
            * / 
           var $; 
           $ =  123 ;
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