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I'm using "Closure Compiler" , when compiling my scripts I spend the following: Before compiling: // ==ClosureCompiler== // @compilation_level SIMPLE_OPTIMIZATIONS // @output_file_name default.js // @formatting pretty_print,print_input_delimiter // ==/ClosureCompiler== var myObj1 = (function() { var undefined; //<----- declare undefined this.test = function(value, arg1) { var exp = 0; arg1 = arg1 == undefined ? true : arg1; //<----- use declare undefined exp = (arg1) ? value * 5 : value * 10; return exp; }; return this; }).call({}); var myObj2 = (function() { this.test = function(value, arg1) { var exp = 0; arg1 = arg1 == undefined ? true : arg1; //<----- without declare undefined exp = (arg1) ? value * 5 : value * 10; return exp; }; return this; }).call({}); Compiled: // Input 0 var myObj1 = function() { this.test = function(b, a) { a = a == void 0 ? true : a; //<----- var c = 0; return c = a ? b * 5 : b * 10 }; return this }.call({}), myObj2 = function() { this.test = function(b, a) { a = a == undefined ? true : a; //<----- var c = 0; return c = a ? b * 5 : b * 10 }; return this }.call({}); With this I believe the question of the use of "void 0 " and "undefined", is there any difference in the use or the two cases are well?. Edit if I define "var undefined" compiled with "void 0 ", if I did not define "undefined" compiled with "undedined. " then not a matter of number of characters between "undefined" and "void 0" Test Edit II: performance, based on this link <a href="http://jsfiddle.net

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