JavaScript variables

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); }