• Understand the differences between variable scope in Ruby and JavaScript
• Use local variables and global variables

In this lesson, we'll introduce the concepts of method scope and variable scope.

Naming variables is hard, but it is important. We need our code to be as descriptive as possible. Any other developer reading over one of our programs should be able to understand what our code does. A big part of this is having variable (and method) names that are sensible and descriptive.

Methods in Ruby create their own scope. "Scope" refers to the areas of your program in which certain data is available to you. Any local variable created outside of a method will be unavailable inside of a method. In addition, local variables created inside of a method (i.e. in between the def and end keywords of a method) 'fall out of scope' once you're outside the method.

Let's take a look at the following example:

name = "Joe"

def greeting(name)
   puts "Hello, #{name}"
end

In this code snippet, we have a variable, name, set equal to a string, "Joe". Then we define a method, #greeting, that takes in an argument of name. Are the name variable that is set equal to "Joe" and the name variable we are using as a parameter (or argument) for our #greeting method definition the same? Let's see.

If we call our greeting method in the following way:

greeting("Sophie")

What do we expect to be outputted to the terminal?

In this case, the above method invocation would puts out Hello, Sophie to the terminal. It is true that we are setting a variable, name, equal to "Joe" in this code snippet. But we are not using that variable anywhere else in our code. The name argument of the #greeting method is just a placeholder. It means: when we call the #greeting method with an argument of, say, "Sophie", set the variable name inside of the method equal to that string argument.

The name variable inside of the #greeting method is different from the name variable that we set equal to "Joe" outside of the method. The #greeting method has its own scope, and variables inside of it don't know about variables outside of it and vice versa.

Think of a method as a castle. The def and end keywords are like the gates that keep out the barbarian hordes, dragons, etc. Let's take a look:

evil_monster = "Bowser"

def princess_peaches_castle
  puts "#{evil_monster} is trying to kidnap Princess Peach!"
end

We've defined the variable evil_monster outside of the method, #princess_peaches_castle. Then, we try to call on the evil_monster variable inside that method. Watch what happens when we invoke the method:

princess_peaches_castle
#=> NameError: undefined local variable or method `evil_monster' for main:Object

The evil_monster variable is out of scope for this method. The method can't access it unless we pass it in as an argument.

In JavaScript, scope rules would allow us to use a variable in the outer scope from our function:

const evilMonster = "Bowser";

function princessPeachesCastle() {
  console.log(`${evilMonster} is trying to kidnap Princess Peach!`);
}

This is a key difference between Ruby and JavaScript: in Ruby, local variables are only available within the scope of the method they are defined in.

If we define our method to accept an argument, we can pass our variable into the method and the method will be able to operate on/use that variable. Let's take a look:

evil_monster = "Bowser"

def princess_peaches_castle(evil_monster)
  puts "#{evil_monster} is trying to kidnap Princess Peach!"
end

princess_peaches_castle(evil_monster)
#=> "Bowser is trying to kidnap Princess Peach!"

The method call, which happens outside the method itself, has access to our evil_monster variable. We can therefore make the value stored in evil_monster available to #princess_peaches_castle by passing it as an argument when we call the method. So now Mario can start his adventure.

So far, we've seen that variables defined outside of methods are not available inside methods (unless we pass them in as arguments). This works the other way around as well: variables defined inside of methods are not available outside of those methods. Let's take a look.

If we define the following method to include a local variable:

def practicing_method_scope
  im_trapped_in_the_method = "You can't access me outside this method!"
end

Trying to access that variable elsewhere in our program, outside of this method, will raise the following error:

im_trapped_in_the_method
#=> NameError: undefined local variable or method `im_trapped_in_the_method' for main:Object

So far, we've encountered one type of variable in Ruby: local variables. They're called "local" because they don't ever leave the scope they're defined in. There are other types of variables in Ruby as well, which can be defined using different naming conventions:

• Local variables: must start with a lowercase letter or an underscore (_): name = "Lucy"
• Global variables: must start with a dollar sign ($): $name = "Lucy"
• Instance variables: must start with an at symbol (@) @name = "Lucy")
• Class variables: must start with two at symbols (@@) (@@name = "Lucy")

We'll discuss instance variables and class variables later along with Object Oriented Programming, but global variables can be useful to see how Ruby treats different variable types with regards to their scope.

For example, we could use a global variable to make the $evil_monster available everywhere in our code:

$evil_monster = "Bowser"

def princess_peaches_castle
  puts "#{$evil_monster} is trying to kidnap Princess Peach!"
end

You should almost never need to use global variables in your code. Having a lot of global variables makes your code unpredictable and challenging to debug, so you should reserve them for special cases (like application-wide configuration).

Remember: A local variable defined inside a method can't leave that method. It is not available to your program outside of the method. A local variable defined outside of a method can only be made available to the code inside the method if you pass that variable in to the method as an argument.