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.

Image of 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.

Getting Started

  1. Fork the Mars rover starter

  2. Write a unit test for each test item listed. shown below.


    One complete test has been created for you as an example.

  3. Use test-driven development (TDD) to create each of the classes described below.

  4. Each class should be defined in its own file, which will be exported and imported as a module.

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 (14, if you do the bonus) for this assignment.


Did you catch the part about only working on ONE test at a time? Do NOT try to write all of the tests at once. Doing so will be inefficient and will cause excessive frustration.

A. Command

Command Class

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 only used assert methods to check for equality. Using assert.throws to verify if a specific error is thrown is a new concept. To learn how to use this new ability of assert, look at the constructor in command.js and look at the test description in command.spec.js. You can also look at the official Node.js assert.throws documentation.
  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 line 12 in command.spec.js to message: 'Oops'. Click "Run" again to verify that the test fails (the error message did not match "Command type required.").
  5. Restore line 12 to message: "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 assert.throws() in this test.
  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.

B. Message

Recall, the role of a message object is to bundle commands to send to the rover.

Message Class

  1. This class builds an object with two properties. constructor(name, commands)
    1. name is a string that is the name of the message.
    2. commands is an array of Command objects.


let commands = [new Command('MODE_CHANGE', 'LOW_POWER'), new Command('STATUS_CHECK')];
let message = new Message('Test message with two commands', commands);

Message Tests

At the same level as command.spec.js, create a file message.spec.js and write the unit tests for the Message class as described below. Remember to use the given phrase as the test description.

Test 4

For this test description, use the text, "throws error if a name is NOT passed into the constructor as the first parameter". Review the first test in command.spec.js for an example of how to write this test.

  1. When you click "Run", the test will likely fail, either because you have no Message class yet or have not written the class to include this feature.

  2. If you haven't done so yet, create a message.js file and add exports and require statements as needed for your modules.


    For help using require to import a class, notice in command.js that the Command class is exported using:

    module.exports = Command;

    In spec/command.spec.js the Command class is imported with this statement:

    const Command = require('../command.js');
  3. Look at the code in command.js. Use that to help you write the Message class in message.js so that your test passes. Refer to the Message Class description above for more details.

Test 5

Use "constructor sets name" as the description. The test confirms that the constructor in the Message class correctly sets the name property in a new message object.

Test 6

Use "contains a commands array passed into the constructor as 2nd argument". This test confirms that the commands property of a new message object contains the data passed in from the Message(name, commands) call.

  1. Hint: Inside this test, you will have to create a commands array, fill it with some Command objects, and pass it into the Message constructor.


You are moving onto the red planet now. Be prepared for fewer instructions.

C. Rover

Rover receives a message object, updates its properties from the message, and returns the results.

Rover Class

This class builds a rover object with a few properties, and it also contains a function outside of constructor to handle updates to its properties.

  1. constructor(position)
    1. position is a number representing the rover's position.
    2. Sets this.position to position
    3. Sets this.mode to 'NORMAL'
    4. Sets default value for generatorWatts to 110
  2. receiveMessage(message)
    1. message is a Message object
    2. Returns an object containing at least two properties:
      1. message: the name of the original Message object
      2. results: an array of results. Each element in the array is an object that corresponds to one Command in message.commands.
    3. Updates certain properties of the rover object
      1. Details about how to respond to different commands are in the Command Types table.


let commands = [new Command('MODE_CHANGE', 'LOW_POWER'), new Command('STATUS_CHECK')];
let message = new Message('Test message with two commands', commands);
let rover = new Rover(98382);    // Passes 98382 as the rover's position.
let response = rover.receiveMessage(message);



   message: 'Test message with two commands',
   results: [
         completed: true
         completed: true,
         roverStatus: { mode: 'LOW_POWER', generatorWatts: 110, position: 98382 }

Rover Tests

Create spec/rover.spec.js and write the following tests. Write the code to make them pass in rover.js. Remember to use the given phrase as the test description.

Test 7

"constructor sets position and default values for mode and generatorWatts". Refer to the Rover Class description above for these default values.

Test 8

"response returned by receiveMessage contains name of message"

Test 9

"response returned by receiveMessage includes two results if two commands are sent in the message"

Test 10

"responds correctly to status check command"

  1. For the STATUS_CHECK command, receiveMessage(message).results includes a roverStatus object describing the current state of the rover object --- mode, generatorWatts, and position. The test should check each of these for accuracy.
  2. See the Rover Command Types table for more details.

Test 11

"responds correctly to mode change command".

  1. The test should check the completed property and rover mode for accuracy.
  2. The rover has two modes that can be passed a values to a mode change command, 'LOW_POWER' and 'NORMAL'.

Test 12

"responds with false completed value when attempting to move in LOW_POWER mode".

  1. The test should check the completed property for accuracy and confirm that the rover position did not change.
  2. Use the Rover Modes table for guidance on how to handle move commands in different modes.

Test 13

"responds with position for move command".

  1. A MOVE command will update the rover's position with the position value in the command.

Rover Command Types

Command Value sent with command Updates to Rover object Result returned
MOVE Number representing the position the rover should move to. position {completed: true}
STATUS_CHECK No values sent with this command. No updates {completed: true, roverStatus: {mode: 'NORMAL', generatorWatts: 110, position: 87382098}} Values for mode, generatorWatts, position will depend on current state of rover.
MODE_CHANGE String representing rover mode (see modes) mode {completed: true}


The response value for completed will be false if the command could NOT be completed.

Rover Modes

Mode Restrictions
LOW_POWER Can't be moved in this state.

Bonus Mission

Add the following test that checks for unknown commands in spec/rover.spec.js.

Test 14

"completed false and a message for an unknown command".

Submitting Your Work

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

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