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"
Edit II: performance, based on this link
IE 8:
typeof: 228ms
undefined: 62ms
void 0: 57ms
Firefox 3.6:
typeof: 10ms
undefined: 3ms
void 0: 3ms
Opera 11:
typeof: 67ms
undefined: 19ms
void 0: 20ms
Chrome 8:
typeof: 3ms
undefined: 5ms
void 0: 3ms
The
void
operator evaluates the givenexpression
and then returnsundefined
.This operator allows inserting expressions that produce side effects into places where an expression that evaluates to undefined is desired.
The void operator is often used merely to obtain the
undefined
primitive value, usually using "void(0)
" (which is equivalent to "void 0
"). In these cases, the global variableundefined
can be used instead (assuming it has not been assigned to a non-default value).
Closure Compiler swaps in void 0
because it contains fewer characters than undefined
, therefore producing equivalent, smaller code.
Re: OP comment
yes, I read the documentation, but in the example I gave, "google closure" in a case using "void 0" and another "undefined"
I believe this is actually a bug in Google Closure Compiler!