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JavaScript variables

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Variables in Javascript

Scoping

  1. Declare all variables with var before assigning to them. Assigning to undeclared variable causes a strict Javascript warning ("redeclaration of var X"; you need to set javascript.options.strict and javascript.options.showInConsole prefs to see it though) and the variable is put in global scope, which is obviously bad.
    Bad example Good example
    function F() {
      // generates a strict JS warning
      x=1;
      G();
      // alerts "2", since x is global.
      alert(x);
    }
    
    function G() {
      x=2;
    }
    function F() {
      var x=1;
      G();
      alert(x); // alerts "1"
    }
    
    function G() {
      var x=2;
    }
    
    
    

  2. Declare each variable exactly once in a function, otherwise you'll get another JS error (redeclaration of var X).
    Good example Bad example
    var i;
    if(condition) {
      for(i=0; i<3; i++)
        dump(i);
    } else {
      for(i=2; i>=0; i--)
        dump(i);
    }
    if(condition) {
      for(var i=0; i<3; i++)
        dump(i);
    } else {
      // generates a JS warning
      for(var i=2; i>=0; i--) 
        dump(i);
    }

    Due to the fact that {} blocks don't introduce a new variable scope (all variables declarations are effectively moved to the start of the enclosing function by interpreter), the following two examples are also valid:

    Kinda ok Works, but may become unmantainable
    if(condition) {
      for(var i=0; i<3; i++)
        dump(i);
    } else {
      for(i=2; i>=0; i--)
        dump(i);
    }
    
    
    if(condition) {
      for(i=0; i<3; i++)
        dump(i);
    } else {
      // generates a JS warning
      for(var i=2; i>=0; i--) 
        dump(i);
    }