Arrays Lab

Learning Goals

• Practice writing arrays
• Practice using destructive methods to manipulate arrays
• Practice using nondestructive methods to manipulate arrays

Introduction

We've learned about how arrays work and about the array methods built in to JavaScript that we can use to manipulate them. Now it's time to practice what we've learned.

If you haven't already, fork and clone this lab into your local environment. Navigate into its directory in the terminal, then run code . to open the files in Visual Studio Code.

Instructions

Open up the test folder and take a look at indexTest.js. Note that some of the names of the functions you will be writing begin with destructively and some don't. This is a clue as to which Array method you will need to use for each function.

Note also that the first test asks for an array called cats, set to an initial value of ["Milo", "Otis", "Garfield"]. In your functions, you will be accessing and manipulating this array.

Near the top of indexTest.js you will see the following:

beforeEach(function () {
  cats.length = 0;

  cats.push("Milo", "Otis", "Garfield");
});

What this code does is reset the array to its original contents before each test is run. The reason we need to do this is because some of your functions will be destructive — they will change the original cats array. This is a problem because it means the input to the remaining functions will be dependent on the outcome of other functions. It also means that the expected return value of a function might change if the tests are run in a different order. This makes it more difficult both to write tests in the first place and to figure out how to get the tests to pass. Resetting the array returns us to a blank slate between tests.

This is also a good illustration of why it's generally good practice to avoid mutating a program's state whenever possible. If we use only nondestructive methods, we have complete control over what's going into and coming out of the function. This makes our programs more robust, easier to maintain, and less prone to bugs.

Remember the workflow:

1. Run the tests using learn test.
2. Read the errors; vocalize what they're asking you to do.
3. Write code; repeat steps 1 and 2 often until a test passes.
4. Repeat as needed for the remaining tests.