javascript

Mimicking Block Scope
Chapter 7: Anonymous Functions
As mentioned previously, JavaScript has no concept of block - level scoping, meaning variables defined
inside of block statements are actually created in the containing function, not within the statement.
Consider the following:
function outputNumbers(count){
for (var i=0; i < count; i++){
alert(i);
}
}
alert(i);
//count
In this function, a for loop is defined and the variable i is initialized to be equal to 0. For languages such
as Java and C++, the variable i would be defined only in the block statement representing the for loop,
so the variable would be destroyed as soon as the loop completed. However, in JavaScript the variable i is
defined as part of the outputNumbers() activation object, meaning it is accessible inside the function
from that point on. Even the following errant redeclaration of the variable won ’ t wipe out its value:
function outputNumbers(count){
for (var i=0; i < count; i++){
alert(i);
}
}
var i; //variable redeclared
alert(i); //count
JavaScript will never tell you if you ’ ve declared the same variable more than once; it simply ignores all
subsequent declarations (though it will honor initializations). Anonymous functions can be used to
mimic block scoping and avoid such problems.
The basic syntax of an anonymous function used as a block scope (often called a private scope ) is as
follows:
(function(){
//block code here
})();
This syntax defines an anonymous function that is called immediately. The function declaration is
enclosed in parentheses to indicate that it ’ s actually a function expression. This function is then called via
the second set of parentheses at the end. If this syntax is confusing, consider the following example:
var count = 5;
outputNumbers(count);
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