Let's talk a bit about scope — a very important concept when dealing with functions. When you create a function, the variables and other things defined inside the function are inside their own separate scope, meaning that they are locked away in their own separate compartments, unreachable from code outside the functions.

The top level outside all your functions is called the global scope. Values defined in the global scope are accessible from everywhere in the code.

JavaScript is set up like this for various reasons — but mainly because of security and organization. Sometimes you don't want variables to be accessible from everywhere in the code — external scripts that you call in from elsewhere could start to mess with your code and cause problems because they happen to be using the same variable names as other parts of the code, causing conflicts. This might be done maliciously, or just by accident.

For example, say you have an HTML file that is calling in two external JavaScript files, and both of them have a variable and a function defined that use the same name:

Both functions you want to call are called greeting(), but you can only ever access the first.js file's greeting() function (the second one is ignored). In addition, an error results when attempting (in the second.js file) to assign a new value to the name variable — because it was already declared with const, and so can't be reassigned.

Note: You can see this example running live on GitHub (see also the source code).

Keeping parts of your code locked away in functions avoids such problems, and is considered the best practice.

It is a bit like a zoo. The lions, zebras, tigers, and penguins are kept in their own enclosures, and only have access to the things inside their enclosures — in the same manner as the function scopes. If they were able to get into other enclosures, problems would occur. At best, different animals would feel really uncomfortable inside unfamiliar habitats — a lion or tiger would feel terrible inside the penguins' watery, icy domain. At worst, the lions and tigers might try to eat the penguins!

The zoo keeper is like the global scope — they have the keys to access every enclosure, to restock food, tend to sick animals, etc.