JavaScript® 24-Hour Trainer

Chapter 6. Scope

In the Try It section of Lesson 5, you used a variable called i to count a for loop's iterations. Most programmers use the i identifier for that exact reason; it's used everywhere and all the time to count loop iterations. So if multiple developers were working on different pieces of the same application, wouldn't all these uses of i conflict with each other?

Thanks to scope, the answer is no. Scope is an important concept in programming, particularly in JavaScript. It's important to understand scope because it determines what variables a function can see and access.

GLOBAL SCOPE

Variables and functions defined outside of a function are created in the global scope. JavaScript's global scope is the topmost level of scope. It persists throughout the entire web page, and any variables and functions created in the global scope are accessible anywhere in the page.

Consider the following code:

var globalVar = "This is a global variable.";

function globalFunction() {
    globalVar = "This changes the value of globalVar";
}

globalFunction();

In this example, a variable called globalVar is created outside of any function; thus, it is a global variable and can be accessed anywhere in the page. Because the globalVar variable is a global variable, the globalFunction() function, which is also defined in the global scope, can access the variable and modify it. Since globalVar is global, its new value can be seen and accessed anywhere in the page; modifying a global variable's value makes ...