Mocking and stubbing in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mock function using Deno's standard testing library.

Deno

import { [1] } from "https://deno.land/std/testing/mock.ts";

const mockFn = [1]();

Drag options to blanks, or click blank then click option'

Aspy

Bfake

Cstub

Dmock

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'stub' instead of 'mock' when creating a mock function.

Importing from the wrong module.

The mock function from Deno's testing library creates a mock function that can be used to replace real functions during tests.

2fill in blank

medium

Complete the code to stub a method on an object using Deno's stub function.

Deno

import { stub } from "https://deno.land/std/testing/mock.ts";

const obj = { greet: () => "Hello" };
stub(obj, "greet", [1]);

Drag options to blanks, or click blank then click option'

A"Hi"

B() => "Hi"

Ctrue

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string directly instead of a function.

Passing a boolean or undefined instead of a function.

The stub function replaces the original method with the provided function. Here, we pass a function that returns "Hi".

3fill in blank

hard

Fix the error in the code to restore the original method after stubbing.

Deno

import { stub } from "https://deno.land/std/testing/mock.ts";

const obj = { greet: () => "Hello" };
const greetStub = stub(obj, "greet", () => "Hi");
// After test
[1]();

Drag options to blanks, or click blank then click option'

AgreetStub.restore()

Bstub.restore()

CgreetStub.restore

Dobj.greet.restore()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to call restore as a function (missing parentheses).

Trying to restore on the original object instead of the stub.

To restore the original method, you must call the restore() method on the stub instance.

4fill in blank

hard

Fill both blanks to create a stub that returns a resolved promise with value 42.

Deno

import { stub } from "https://deno.land/std/testing/mock.ts";

const api = { fetchData: async () => 0 };
stub(api, "fetchData", [1]);

const result = await api.fetchData(); // result should be 42

// result === [2]

Drag options to blanks, or click blank then click option'

Aasync () => 42

B42

CPromise.resolve(42)

Dasync () => Promise.resolve(42)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 42 directly without async or promise.

Returning a promise but not marking the function async.

The stub replaces fetchData with an async function returning a resolved promise with 42. The result is the number 42.

5fill in blank

hard

Fill all three blanks to create a mock function, call it with argument 5, and check the call count.

Deno

import { mock } from "https://deno.land/std/testing/mock.ts";

const fn = [1]();

fn([2]);

const calls = fn.[3];

// calls should be 1

Drag options to blanks, or click blank then click option'

Amock

B5

Ccalls.length

DcallCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using calls.length instead of callCount.

Passing argument as a string instead of number.

The mock function creates a mock function. Calling it with 5 records the call. The callCount property shows how many times it was called.

Practice

1. What is the main purpose of mocking and stubbing in Deno testing?

easy

A. To speed up the execution of the entire program

B. To replace parts of code with fake versions for easier testing

C. To permanently change the behavior of functions in production

D. To automatically generate user interface components

Solution

1. Step 1: Understand mocking and stubbing

   Mocking and stubbing are techniques used to replace parts of code with fake versions to isolate and test specific behaviors.

2. Step 2: Identify the purpose in testing context

   They help test code without relying on real dependencies, making tests simpler and more reliable.

3. Final Answer:

   To replace parts of code with fake versions for easier testing -> Option B

4. Quick Check:

   Mocking purpose = Replace code parts [OK]

Hint: Mocking replaces code parts to isolate tests [OK]

Common Mistakes:

• Thinking mocking speeds up the whole program
• Believing mocks change production code permanently
• Confusing mocking with UI generation

2. Which of the following is the correct way to import the stub function from Deno's standard testing library?

easy

A. import { stub } from "https://deno.land/std/testing/stub.ts";

B. import stub from "https://deno.land/std/testing/mock.ts";

C. import { stub } from "https://deno.land/std/testing/mock.ts";

D. import * as stub from "https://deno.land/std/testing/mock.ts";

Solution

1. Step 1: Recall the correct import path and syntax

   The stub function is exported from "https://deno.land/std/testing/mock.ts" and must be imported using named import syntax.

2. Step 2: Check each option

   import { stub } from "https://deno.land/std/testing/mock.ts"; uses named import from the correct path. import stub from "https://deno.land/std/testing/mock.ts"; uses default import which is incorrect. import { stub } from "https://deno.land/std/testing/stub.ts"; uses wrong file name. import * as stub from "https://deno.land/std/testing/mock.ts"; imports all as an object, not the function directly.

3. Final Answer:

   import { stub } from "https://deno.land/std/testing/mock.ts"; -> Option C

4. Quick Check:

   Correct import = named from mock.ts [OK]

Hint: Use named import from mock.ts for stub [OK]

Common Mistakes:

• Using default import instead of named import
• Wrong file path for stub
• Importing everything instead of just stub

3. Given the following code snippet, what will be the output?

import { stub } from "https://deno.land/std/testing/mock.ts";

const obj = {
  greet() {
    return "Hello";
  }
};

const stubGreet = stub(obj, "greet", () => "Hi there!");
console.log(obj.greet());
stubGreet.restore();
console.log(obj.greet());

medium

A. "Hello" then "Hello"

B. "Hi there!" then "Hi there!"

C. Runtime error due to incorrect stub usage

D. "Hi there!" then "Hello"

Solution

1. Step 1: Understand stub behavior on method

   The stub replaces obj.greet with a new function returning "Hi there!" temporarily.

2. Step 2: Analyze console outputs

   First console.log calls stubbed method, outputting "Hi there!". After restore(), original method returns "Hello" on second log.

3. Final Answer:

   "Hi there!" then "Hello" -> Option D

4. Quick Check:

   Stub replaces then restores method [OK]

Hint: Stub changes method until restore is called [OK]

Common Mistakes:

• Expecting original method output before restore
• Thinking stub permanently changes method
• Confusing stub with spy that tracks calls

4. What is the error in the following Deno test code using stub?

import { stub } from "https://deno.land/std/testing/mock.ts";

const obj = {
  value() {
    return 42;
  }
};

const stubValue = stub(obj, "val", () => 100);
console.log(obj.value());
stubValue.restore();

medium

A. Trying to stub a non-existent method 'val' on obj

B. Missing import statement for stub

C. restore() called before stub is created

D. Incorrect arrow function syntax in stub

Solution

1. Step 1: Check stub target method name

   The stub tries to replace method named "val" but obj only has "value" method.

2. Step 2: Understand stub behavior on invalid method

   Stubbing a non-existent method causes an error or does not work as expected.

3. Final Answer:

   Trying to stub a non-existent method 'val' on obj -> Option A

4. Quick Check:

   Stub method must exist on object [OK]

Hint: Stub only existing methods on objects [OK]

Common Mistakes:

• Using wrong method name in stub
• Forgetting to import stub
• Calling restore too early

5. You want to test a function that calls fetchData() which normally fetches data from the internet. Using Deno's stub, how can you replace fetchData to return a fixed value { success: true } only during the test, and ensure the original fetchData is restored after the test?

hard

A. Use const stubFetch = stub(obj, "fetchData", () => ({ success: true })); then call stubFetch.restore() after test

B. Replace fetchData globally without restore, returning fixed value

C. Use stub(obj, "fetchData", async () => Promise.resolve({ success: true })); but forget to restore

D. Call stub(obj, "fetchData", () => { return true; }); without storing stub

Solution

1. Step 1: Create a stub for fetchData returning fixed value

   Use stub on the object containing fetchData, replacing it with a function returning { success: true }.

2. Step 2: Restore original method after test

   Store the stub in a variable and call restore() after the test to revert changes.

3. Final Answer:

   Use const stubFetch = stub(obj, "fetchData", () => ({ success: true })); then call stubFetch.restore() after test -> Option A

4. Quick Check:

   Stub with restore keeps tests clean [OK]

Hint: Always restore stub after test to keep original code [OK]

Common Mistakes:

• Not restoring stub after test
• Replacing globally without control
• Returning wrong stub value type