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Spies, Mocks, & Stubs

Objectives

  1. Learn the difference between spies, mocks, and stubs
  2. Write tests for code with dependencies

Exercise A

In this exercise, you are given some code that already has partial coverage.

Edit calculator.ts; remove the lines about istanbul ignore on line 28 & 29 that look like this:

/* istanbul ignore next */

By removing these lines, the class defined below it will begin to count against the code coverage reports. You should notice your code coverage is no longer at 100%. Make sure your watcher is running (grunt watch)

  1. Implement the remaining tests and fix any bugs you find in the process.
  2. Add NegateExpression (-n) - inverts the provided Expression. For example, NegateExpression(NumberExpression(-3)) would evaluate to 3.
  3. Add PowerExpression (2^n) - takes the given Expression and sets it as the exponent of 2. For example, PowerExpression(NumberExpression(3)) would evaluate to 8
  4. Make sure your code coverage is constantly at 100%.

Exercise B

In most of your previous tests, you probably used the NumberExpression class. This might make sense because in this case, NumberExpression is very straight forward. The problem with this approach is that if there is a bug in NumberExpression, all of your subsequent tests break.

Replace the contents of NumberExpression.eval() so it always returns a random number between 1 - 3:

Math.ceil(Math.random() * 3)

Notice how most of your tests are now broken, and inconsistently at that! Fix all your tests so that each passes regardless of what Expression implementation is passed into it. Try approaches varying between mocks, stubs, and dummy implementations of Expression. When done, revert the "bug" introduced to NumberExpression.eval().

Exercise C

In this last example, we will examine using external dependencies. In this specific case, we will treat JavaScript standard library Math as one: you can't modify it, didn't write it, but need its behavior.

Add the following new Expression type and write tests for it:

  • RandomExpression (random(n)) - return a random integer between 0 and Expression, inclusively. For example, RandomExpression(NumberExpression(3)) would evaluate to a random number 0 - 3

Note that this last Expression implementation's tests will absolutely require that you stub the Math.random method.