Access a global variable in a browser environment:

myglobal = "hello"; // antipattern

console.log(myglobal); // "hello"

console.log(window.myglobal); // "hello"

console.log(window["myglobal"]); // "hello"

console.log(this.myglobal); // "hello"  

1. The problem with Globals
   • Naming collisions
     1. Code not written by developers

        ? A third-party JavaScript library

        ? Scripts from an advertising partner

        ? Code from a third-party user tracking and analytics script

        ? Different kinds of widgets, badges, and buttons

     2. Implied globals

        meaning that any variable you don‘t declare becomes a property of the global object.

        Solution - Use var to declare variable inside the function.

        

        function sum(x, y) {
        
           var result = x + y;
        
           return result;
        
        }
           
        
        // antipattern, do not use
        
        function foo() {
        
            var a = b = 0; // a is local but b becomes global
        
            // ...
        
        }
           
        
        // The right way
        
        function foo() {
        
           var a, b;
        
           // ...
        
           a = b = 0; // both local
        
        } 

        
   • portability

     Code to run in different environments (hosts), it‘s dangerous to use globals because you can accidentally overwrite a host object that doesn‘t exist in your original environment (so you thought the name was safe to use) but which does in some of the others.

2. Side Effects when Forgetting var

   Difference between implied globals and explicitly defined ones—the difference is in the ability to undefine these variables using the delete operator

   ? Globals created with var(those created in the program outside of any function) cannot be deleted.

   ? Implied globals created without var(regardless if created inside functions) can be deleted.

   

   // define three globals
   
   var global_var = 1;
   
   global_novar = 2; // antipattern
   
   (function () {
   
       global_fromfunc = 3; // antipattern
   
   }());
   
   // attempt to delete
   
   delete global_var; // false
   
   delete global_novar; // true
   
   delete global_fromfunc; // true
   
   // test the deletion
   
   typeof global_var; // "number"
   
   typeof global_novar; // "undefined"
   
   typeof global_fromfunc; // "undefined"  

   
     
3. Access to the Global Object

   Access the global object without hard-coding the identifier window, you can do the following from any level of nested function scope:

   var global = (function () {
   
      return this;
   
   }());    

4. Single var Pattern

   ? Provides a single place to look for all the local variables needed by the function

   ? Prevents logical errors when a variable is used before it‘s defined (see "Hoisting: A Problem with Scattered vars" )

   ? Helps you remember to declare variables and therefore minimize globals

   ? Is less code (to type and to transfer over the wire)

   

   function func() {
      var a = 1,
      b = 2,
      sum = a + b,
      myobject = {},
      i,
      j;
      // function body...
   } 

   

   Note: all uninitialized and declared variables are initialized with the value undefined

   

   function updateElement() {
       var el = document.getElementById("result"),
   
       style = el.style;
   
       // do something with el and style...
   }    

   
5. Hoisting: A problem with Scattered vars

   JavaScript enables you to have multiple var statements anywhere in a function, and they all act as if the variables were declared at the top of the function.  

   

   // antipattern
   
   myname = "global"; // global variable
   
   function func() {
   
       // same as -> var myname = undefined;
   
       alert(myname); // "undefined"
   
       var myname = "local";
   
       alert(myname); // "local"
   
   }
   
   func(); 

   

JavaScript Patterns 2.2 - Minimizing Globals,布布扣,bubuko.com

JavaScript Patterns 2.2 - Minimizing Globals