Assignment #3: Mars Rover

This task puts your unit testing, modules making, and exception handling knowledge to use by writing tests and classes for the Mars rover named Curiosity.

Curiosity rover taken by the rover on Mars.

Selfie of Curiosity on Mars.

You will create a simulation for issuing commands to Curiosity. The idea is to create a command at mission control, convert that command into a message send it to the rover, then have the rover respond to that message.

We will provide descriptions of the required features you need to implement in three separate classes:

  1. Command: A type of object containing a commandType property. commandType is one of the given strings in the table below. Some commandTypes are coupled with a value property, but not all. Every Command object is a single instruction to be delivered to the rover.

  2. Message: A Message object has a name and contains several Command objects. Message is responsible for bundling the commands from mission control and delivering them to the rover.

  3. Rover: An object representing the mars rover. This class contains information on the rover's position, operating mode, and generatorWatts. It also contains a function, receiveMessage that handles the various types of commands it receives and updates the rover's properties.

In true TDD form, you will be asked to first write the appropriate units tests for these features, then write the code in the given class to pass those tests.

Get the Starter Code

Fork this replit.

How-To TDD

Recall that in TDD, you write the test for a given behavior before you code the actual function. Feel free to review the Test/Code cycle while you work on this project.

  1. Focus on one test at a time.

  2. Write the test and then create the code to make it pass.

  3. Only write the minimum amount of code needed to make the test pass.

  4. There are some constraints on how you can implement these features. A description of each class is below.

Each numbered item describes a test. You should use the given phrases as the test descriptions when creating your it statements. You must create 13 tests for this assignment.


Do NOT try to write all of the tests at once. Doing so will be inefficient and will cause excessive frustration.


Command Class Description

We'll follow TDD practices for the creation of Message and Rover, but for this class, Command, we've provided the functionality. Command is already written for you and you do not need to modify it to write passing tests. Open up and examine the file command.js.

  1. This class builds an object with two properties. constructor(commandType, value)

    1. commandType is a string that represents the type of command. We will go over the details of the types when we get to the Rover class and tests. At this time, note that a command type will be one of the following: 'MODE_CHANGE', 'MOVE', or 'STATUS_CHECK'.

      1. To peek ahead at the full functionality of these types, refer to Command Types table.

    2. value is a value related to the type of command.


let modeCommand = new Command('MODE_CHANGE', 'LOW_POWER');
let moveCommand = new Command('MOVE', 12000);

'MODE_CHANGE' and 'MOVE' are passed in as the commandType

'LOW_POWER' and 12000 are passed in as the value. Different command types require different kinds of values. 'STATUS_CHECK' takes no value.

Don't worry about the mode options for now. To peek ahead, see Rover Modes table.

Now that we've gone over the class, let's check out the tests.

Command Tests

To begin, open and examine spec/command.spec.js. One test has been created for you. When a user creates a new Command object from the class, we want to make sure they pass a command type as the first argument.

Test 1

Note that the test description reads, "throws error if a command type is NOT passed into the constructor as the first parameter".

  1. So far, you have many used expectations to check for equality. In the chapter on exceptions, we shared an example of how we might use an expectation to check if an exception is thrown. Refer back to that example for guidance on the syntax.

  2. Click "Run" to verify that the test passes. Next, comment out lines 4-6 in command.js. Click "Run" again to verify that the test fails (the expected error is not thrown when the Command class is called).

  3. Restore lines 4-6 to throw Error("Command type required.");.

  4. Change 'Command type required.' on line 9 in command.spec.js to 'Oops'. Click "Run" again to verify that the test fails (the error message did not match "Command type required.").

  5. Restore line 9 to 'Command type required.'.

Test 2

Create a second Command test using, "constructor sets command type" as the description. This test checks that the constructor in the Command class correctly sets the commandType property in the new object.

  1. Without editing, command.js contains the correct code. Click "Run" to verify that the first and second tests both pass.

  2. You do not need to use expect().toThrow().

  3. You may not need to know the specific types of commands to write this test.

Test 3

Code a third test using, "constructor sets a value passed in as the 2nd argument" as the description. This test checks that the constructor correctly sets the value property in the new object.

  1. You may not need to know a proper value in order to write this test.

Click "Run" to verify that all 3 command tests pass.


As you move through the remaining instructions, the amount of guidance will decrease. Refer to your earlier, passing tests to help you construct new tests and passing code.

Great job, astronaut! When you are ready to keep going, check out Part 2!

Submitting Your Work

Once your Rover class is completed and you have written 13 passing specs, you are ready to submit.


Make sure that you did not edit either studentgrading.spec.js, grading.js, or any file in the helpers directory inside spec. Changes to these files could effect your grade.

In Canvas, open the Mars Rover assignment and click the "Submit" button. An input box will appear.

Copy the URL for your repl and paste it into the box, then click "Submit" again.