Task 2: Command Class

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.

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.
  2. To peek ahead at the full functionality of these types, refer to the Command Types Table .
  3. 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 the 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 used many 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. Run the command npm test within your terminal to verify that the test passes. Next, comment out lines 4-6 in command.js. Run npm test 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’. Run npm test 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, it contains the correct code. Run npm test 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. You may not need to know a proper value in order to write this test.

Run npm test 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 the Message Class !