what's the difference between those two?

Question

  var test = function(x) {
         return x + 2;
    };

and I can use test(3)

but when I use

  var test = (function(x) {
         return x + 2;
    })();

I cannot use test(3)

why?

because in the second example you are defining test as the return value, not the function — atmd, Jan 23 '15 at 15:13
Because () executes function in runtime immediately, so test gets the result of function with no arguments (NaN). Then you are trying to run NaN as a function — Rax Wunter, Jan 23 '15 at 15:14
You could do `var test = (function(x) { return x + 2; })(3);`, but that defeats the point of the function. — Andy, Jan 23 '15 at 15:15

score 0 · Answer 1 · answered Jan 23 '15 at 15:14

The second code snippet is an example of an IIFE - Immediately-Invoked Function Expression.

The result of the expression inside the first set of parens defines a function that takes one parameter (named x). The second set of parens immediately invokes it, returning the result. Since you passed no arguments when invoking it, x is undefined so it evaluates the expression undefined + 2 and returns NaN.

score 0 · Answer 2 · answered Jan 23 '15 at 15:14

0

In the first case you is defining a function, so you can do test(3)

In the second case you are executing a function, so, probably, you are getting NaN as the result.

answered Jan 23 '15 at 15:14

Victor

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score 0 · Answer 3 · answered Jan 23 '15 at 15:22

In the second example, you're taking advantage of JavaScript closures. Closures allow you to control scoping and namespacing, and to declare private methods and variables.

Consider the following:

var test = (function() {
 var privateVar1;
 var privateVar2;

 function privateFunction() {
 }

 return {
    publicFunction1: function() {
    },
    publicFunction2: function() {
    }
 };
})();

test is now equal to the following:

{ publicFunction1: function() {}, publicFunction2: function() {} }

The function is immediately invoked after being declared, at which point x does not have a value. Initialize x before the function declaration (outside of its scope) to fix the problem you're experience:

var x = 10;
var test = (function(x) {
         return x + 2;
    })();

what's the difference between those two?

3 Answers3