Mastering JavaScript

Chapter 2
The (function foo(){ }) statement as an expression means that the
identifier foo is found only in the scope of the foo() function, not in the outer
scope. Hiding the name foo in itself means that it does not pollute the enclosing
scope unnecessarily. This is so useful and far better. We add () after the function
expression to execute it immediately. So the complete pattern looks as follows:
(function foo(){ /* code */ })();
This pattern is so common that it has a name: IIFE, which stands for Immediately
Invoked Function Expression. Several programmers omit the function name when
they use IIFE. As the primary use of IIFE is to introduce function-level scope, naming
the function is not really required. We can write the earlier example as follows:
var a = 1;
(function() {
var a = 2;
console.log( a ); // 2
})();
console.log( a ); // 1
Here we are creating an anonymous function as IIFE. While this is identical to the
earlier named IIFE, there are a few drawbacks of using anonymous IIFEs:
• As you can't see the function name in the stack traces, debugging such code
is very difficult
• You cannot use recursion on anonymous functions (as we discussed earlier)
• Overusing anonymous IIFEs sometimes results in unreadable code
Douglas Crockford and a few other experts recommend a slight variation of IIFE:
(function(){ /* code */ }());
Both these IIFE forms are popular and you will see a lot of code using both these
variations.
You can pass parameters to IIFEs. The following example shows you how to pass
parameters to IIFEs:
(function foo(b) {
var a = 2;
console.log( a + b );
})(3); //prints 5
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