Hoisting

Hoisting happens under the covers in JavaScript. When a variable declared with the var keyword is declared, the declaration is moved to the top of the variable's scope. The initialization still occurs in the same location that you entered it. This means that these variables can be used before they're declared. However, if you use the variable before it's been declared and/or initialized, the value will be undefined and your application may produce a bug because of this.

Something similar happens with functions. You can use functions in JavaScript before they're defined because of hoisting.

For more information on this topic, check out this blog post.