Roll Your Own Mock Framework

Level: Advanced 60–90 min

Concepts: MockingDesign PatternsBoundariesIncremental Design

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Build a basic mock object framework that lets developers create test doubles, set up return values, and verify method calls. Understanding how mocks work under the hood makes you better at using them.

Inspired by Jason Gorman’s Codemanship kata.

Requirements

Step 1: Create a Mock

Given an interface or class, create a mock instance that implements the same contract.

interface Calculator {
  add(a, b): number
  subtract(a, b): number
}

mock = createMock(Calculator)

The mock should accept any method call without throwing.

Step 2: Stub Return Values

Configure a mock to return specific values when methods are called with specific arguments.

when(mock.add(2, 3)).thenReturn(5)

result = mock.add(2, 3)  // returns 5
result = mock.add(1, 1)  // returns undefined/null (no stub configured)

Step 3: Verify Invocations

After exercising the system under test, verify that expected methods were called.

mock.add(2, 3)

verify(mock.add).wasCalledWith(2, 3)  // passes
verify(mock.add).wasCalledWith(1, 1)  // fails with meaningful message
verify(mock.subtract).wasCalled()      // fails — never called

Step 4: Meaningful Failure Messages

When verification fails, report what was expected vs what actually happened:

Expected: add(1, 1) to be called
Actual: add was called with (2, 3)

Test Cases

Scenario	Expected
Create mock from interface	Mock instance created, methods callable
Call unstubbed method	Returns null/undefined, no error
Stub return value	Returns configured value for matching args
Stub with different args	Each arg set returns its own value
Verify called method	Passes
Verify uncalled method	Fails with message
Verify wrong arguments	Fails with expected vs actual message
Verify call count	wasCalledTimes(2) passes if called twice

Bonus

• Support wasCalledTimes(n) — verify exact call count
• Support wasNeverCalled() — verify a method was not invoked
• Support argument matchers — verify(mock.add).wasCalledWith(anyNumber(), 3)
• Support thenThrow(error) — mock throws instead of returning
• Support call ordering — verify method A was called before method B

Hint

The core mechanic is a proxy that records all method calls (name + arguments) in a list. Stubbing adds entries to a return-value lookup. Verification searches the recorded calls. Start with recording calls and verifying them — add stubbing after.