Methods

Learning Goals

Understand the similarities between methods in Ruby and functions in JavaScript
Identify key differences between methods and functions
Define methods with parameters
Call methods and use their return value

Introduction

One of the first things you likely learned in JavaScript was how to write functions. In this lesson, you'll get practice writing methods in Ruby to see the difference between Ruby methods and JavaScript functions.

Ruby Method Syntax

To start, let's try re-writing this JavaScript function in Ruby:

function myFunction(param) {
  console.log("Running myFunction");
  return param + 1;
}

As a quick recap of the syntax here:

The function keyword identifies this code as a function.
myFunction is a variable name we can use to refer to the function from elsewhere in our code, written in camel case by convention.
The parentheses () after the function name give a space where we can define parameters for our function.
param is the variable name given to our function's parameter; it will be assigned a value when the function is invoked and passed an argument.
To define the body of the function, we use curly brackets ({ }).
console.log is a method that will output information to the terminal; remember, this is different from a function's return value.
The return keyword is needed when we want our function to return a value when it is called; in this case, it will return a value of whatever the param variable is plus one.

To actually run the code inside the function, we must invoke it:

const myFunctionReturnValue = myFunction(1);
// => "Running myFunction"
console.log(myFunctionReturnValue);
// => 2

Here, we're calling the function myFunction with an argument of 1. We are then assigning the return value of myFunction to a new variable, myFunctionReturnValue.

If we wanted to write a method in Ruby with similar functionality, here's how it would look:

def my_method(param)
  puts "Running my_method"
  param + 1
end

There are a few key differences in the syntax here:

Use the def keyword to identify this code as a method.
Write the name of the method in snake case (by convention).
Parameters are still defined in parentheses, after the method name.
Instead of curly parentheses, use the end keyword to identify where the method ends.
In Ruby, whatever the last line of a method is will be its return value. You can use the return keyword to explicitly identify the return value of a method, but Rubyists tend to rely on the implicit return instead.

Run IRB, and copy/paste the method definition above into your IRB session. Then, run the method:

my_method_return_value = my_method(1)
# Running my_method
# => 2
my_method_return_value
# => 2

When the #my_method method is called, you'll see the output from the #puts method in the terminal, followed by the return value. The return value, 2, is then saved to the variable my_method_return_value.

Why are they called methods in Ruby as opposed to functions? If you recall from Object Oriented JavaScript, the difference between a method and a function is that a method is a special sub-category of functions that must be called on some object. In Ruby, every method we define — even when it's not defined explicitly "on an object" or as part of a class definition — is still actually defined on a built-in Ruby object: the global main object, which you'll see referenced in error messages like "NameError (undefined local variable or method 'my_var' for main:Object)". main is roughly equivalent to the global object in JavaScript (the window in the browser); you can see for yourself by entering IRB and typing self. Read more about Ruby's Main Object here if you're curious!

Optional Parentheses

Just like in JavaScript, you can invoke a method by placing parentheses at the end and passing in values as arguments, as in the example above. In Ruby though, parentheses are optional:

my_method 1
# Running my_method
# => 2

This syntax is a point of confusion for developers new to Ruby, since it can make it a bit less clear when using code whether you're invoking a method or using a variable. Consider the following method:

def say_hello()
  "hello!"
end

Parentheses are optional when defining the method, so we could also write this method like so:

def say_hello
  "hello!"
end

The convention in Ruby is to use parentheses as part of a method definition when it takes parameters, and to omit them when it has no parameters.

You could run this method by placing parentheses at the end, like you would in JavaScript:

say_hello()
# => "hello"

But you can also run this method without parentheses:

say_hello
# => "hello"

In certain contexts, removing parentheses from the method calls can make your code more pleasant to look at. You might also see some Domain Specific Languages (DSLs) that prefer to omit parentheses. You've probably already seen a little bit of RSpec's DSL in the test files, for example:

describe "MyRubyThing" do
  it "runs" do
    # test here
  end
end

#describe and #it are just methods, so the above could have been written:

describe("MyRubyThing") do
  it("runs") do
    # test here
  end
end

But I think you'll agree that it looks nicer (and is easier to read) without the parentheses.

As you're getting started, just keep the fact that parentheses are optional in the back of your mind, and remember to ask yourself the question when looking at code: "Is this a variable, or is this a method?"

Method Arguments

JavaScript allows you to define functions that expect a certain number of arguments, but will still run your code even if you don't pass in the expected number when you invoke the function. This can lead to some unexpected behavior in your JavaScript applications.

Consider the following:

function sayHi(name) {
  console.log(`Hi there, ${name}!`);
}

sayHi();

What do you think will happen when this code runs? Will it throw an error? Print something to the console? If so, what? Try running it in the browser to find out.

Unfortunately for JavaScript developers, bugs like these are hard to identify because they can only be found by testing our code and looking for unexpected behavior.

In Ruby, thankfully, when we run a method without passing in the required arguments it will give us an error message:

def say_hi(name)
  puts "Hi there, #{name}!"
end

say_hi
# => ArgumentError (wrong number of arguments (given 0, expected 1))

Error messages like this are a good thing for us as developers, because it ensures that we are using methods as they are intending to be used, rather than trying to "fail gracefully" like JavaScript does.

Default Arguments

We can fix the behavior of our JavaScript function above by providing a default argument: a value that will be used if we don't explicitly provide one.

function sayHi(name = "friend") {
  console.log(`Hi there, ${name}!`);
}

sayHi();
// => "Hi there, friend!"
sayHi("Sunny");
// => "Hi there, Sunny!"

Ruby also lets us provide default arguments:

def say_hi(name = "Rubyist")
  puts "Hi there, #{name}!"
end

say_hi
# => "Hi there, Rubyist!"

say_hi "Sunny"
# => "Hi there, Sunny!"

Return Values

You can categorize all functions that you write as generally useful for one (or both) of these things:

What return value they have
What side effects they have (what other parts of the application they change; or what they output to the terminal; or what they write to a file; etc)

Writing output to the terminal using console.log or #puts is a side effect of a function: it's distinct from the function's return value.

Consider these two JavaScript functions:

function addAndLog(num1, num2) {
  console.log(num1 + num2);
}

function addAndReturn(num1, num2) {
  return num1 + num2;
}

const sum1 = addAndLog(2, 2);
const sum2 = addAndReturn(2, 2);

What do you expect the values of sum1 and sum2 to be? What output would you expect to see in the console if you ran this code?

Since addAndLog doesn't use the return keyword, the value of sum1 is undefined. We're only using addAndLog for its side effect of logging output to the terminal. sum2, on the other hand, will have a value of 4, since we are using addAndReturn for its return value.

Think of it this way: methods are like vending machines. When you use a vending machine you just put in two arguments, the number (C7) and your money. We already know how to use arguments, but then your vending machine might do two things. One, it will make a noise saying that everything worked, beep beep. Then it gives you the soda. The soda is the return type. But those beeps? Are you able to do anything with them? Nope! That's like #puts: it just tells you stuff and then goes into the ether! Gone forever.

Every method in Ruby returns a value by default. This returned value will be the value of the last statement:

def add_and_log(num1, num2)
  puts num1 + num2
end

def add_and_return(num1, num2)
  return num1 + num2
end

sum1 = add_and_log(2, 2)
# => nil
sum2 = add_and_return(2, 2)
# => 4

The return value of the #add_and_log method is nil, because #puts returns nil.

Say you're the best painter in the world, Bob Ross. To make a method that just prints your name and returns nil, you could write:

def print_name
  puts "Bob Ross"
end

To write a method that returns your name but doesn't print anything, you could write:

def return_name
  "Bob Ross"
end

To both print and return your name, you could write:

def print_and_return_name
  puts "Bob Ross"
  "Bob Ross"
end

If you accidentally switched the order of the lines inside the method:

def broken_print_and_return_name
  "Bob Ross"
  puts "Bob Ross"
end

The method would instead print "Bob Ross" and return nil. This is because the last line that was evaluated was puts ... and the return value of a #puts is always nil.

The Return Keyword

There is one other way to return a value from a method, and that is to use the return keyword.

Let's take a look:

def stylish_painter
  best_hairstyle = "Bob Ross"
  return "Jean-Michel Basquiat"
  best_hairstyle
end

stylish_painter

What do you expect the return value of the above method to be? Go into IRB, copy and paste the above method and call it.

You may have expected the return value to be Bob Ross. His name is the last line of the method. However, the return value of the above method is actually Jean-Michel Basquiat! The return keyword will disrupt the execution of your method, and prevent Ruby from running any lines of code after the return keyword.

The explicit use of the return keyword is generally avoided by many Rubyists, but there are instances where you might want to use return instead of relying on implicit returns; for example, if we wanted to use a guard clause and exit a function early if a certain condition isn't met:

def reverse_name(name)
  if name.class != String
    return nil
  end

  name.reverse
end

reverse_name("Bob Ross")
# => "ssoR boB"
reverse_name(123)
# => nil

Instructions

In the js/index.js file, there are four functions defined in JavaScript. Your job is to recreate the functionality of those functions by writing methods in Ruby that will accomplish the same thing.

Write your code in methods.rb. Run learn test, and use the tests along with the code in js/index.js to guide your work.

Define a method #greet_programmer that takes no arguments. It should output the string "Hello, programmer!" to the terminal with #puts.
Define a method #greet that takes one argument, a name. It should output the string "Hello, name!" (with "name" being whatever value was passed as an argument) to the terminal with #puts.
Define a method #greet_with_default that takes one argument, a name. It should output the string "Hello, name!" (with "name" being whatever value was passed as an argument) to the terminal with #puts. If no arguments are passed in, it should output the string "Hello, programmer!".
Define a method #add that takes two numbers as arguments and returns the sum of those two numbers.
Define a method #halve that takes one number as an argument and returns the that number's value, divided by two. If the argument is not an integer, it should return nil and not throw an error.

Conclusion

Ruby's method syntax has a few things that make them distinct from JavaScript functions. In particular, make sure you pay attention to the implicit return value of Ruby methods, and always call methods with the right number of arguments to avoid errors. You should also keep in mind that parentheses are optional when calling methods.

Resources

Writing Ruby Methods

lorna-chepkirui/phase-3-methods