8 Top Unit Testing Interview Questions (With Sample Answers)

By Indeed Editorial Team

Published 16 May 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

Unit testing is an important process that developers use to ensure a code meets quality standards before being launched. It ensures developers write better code more efficiently, and it saves time and money throughout a product development life cycle. If you're seeking work in the IT industry, it may benefit you to learn more about unit testing questions for interviews. In this article, we explain what unit testing is, list various unit testing interview questions that interviewers ask and give sample responses.

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What are unit testing interview questions?

Unit testing interview questions are questions a hiring manager or recruiter may ask you to assess your technical knowledge about unit tests. Unit testing is a process in the development of software where the smallest testable parts or units of an application are individually tested. Software developers or Quality Assurance (QA) staff usually do this test during the development stage of an application. This process is important because it helps you detect early flaws in your code. The main purpose of unit testing is to isolate written code and run it to see if it works as it's designed to.

This testing process is a component of test-driven development (TDD), which is a software development method that uses a detailed approach to building a product through continual testing and revision. Unit testing is usually isolated to make sure units don't rely on any external codes or functions. The testing process is either automated or done manually by a QA developer. It's generally the first level of software testing and developers perform this test before other methods, such as integration testing.

Related: What Does a QA Engineer Do? (And How to Become One)

How unit testing works

A unit test has three main stages. In the first step, you prepare and review the unit test before it begins. During the second step, you create the test cases and scripts. The third step is where you run the software code test. The TDD method requires developers to begin by writing failing unit tests. They then rewrite the code and apply it to the application repeatedly until the test passes.

For this process, a developer tests all the test cases independently in an isolated environment to ensure there are no dependencies in the code. The developer writes their code criteria to verify each test case, and they use a test framework to identify and report failed tests. Unit testing only involves the characteristics vital to the unit's performance during a test. When you complete testing for all the units in a program and they're error-free and working efficiently, then you can conduct integration testing as the next step. Software developers usually perform unit tests frequently.

Related: Top 20 Software Tester Skills and How to Develop Them

8 interview questions about unit testing

Hiring managers frequently ask unit testing interview questions when conducting interviews for positions such as software developer or QA engineer or analyst. Below are eight commonly asked questions with sample answers:

1. What is your understanding of unit testing case phases?

Interviewers use this question to find out if you know about the unit testing process you can apply for each standard case phase. As you answer, it's important to communicate your knowledge and give specific descriptions.
Example: "Generally, unit testing cases have three standard phases. The first one is where we set the values and conditions for the specific software application we're testing. After this is the second phase, where we run a test on the software and then we go to phase three where we analyse the test results. I have three years of experience running these phases as I conduct unit tests."

2. Can you describe the difference between function testing and acceptance testing?

Interviewers may ask this question so that you can illustrate your knowledge about the different unit tests and distinguishing factors. As you respond, define the function test and the acceptance test and the different phases and steps for each test. Make sure you give detailed and informative answers to the panel as this displays your experience.

Example: "Function testing verifies whether the software application under development is efficient and meets the company requirements. Acceptance testing validates whether an application solves the problem it's been designed to address. The two tests differ in functionality, but they're both necessary for checking whether a software application does what it's been made to do. In my previous experiences, I have run both tests on various applications and I am well acquainted with how they work."

3. How can you describe mocking in relation to unit testing?

Panels use this question to find out whether you know the technical terms used by developers. In your answer, you can give a description of what mocking is and your experience working with this method.

Example: "Mocking is a process in unit testing that observes and examines software code structures and their behaviour. The purpose of this is to identify any software dependencies. I have frequently applied mocking to software I'm testing to help me simulate the behaviours of different real objects and replace them with my simulations. This is helpful where using real objects may be impractical in the unit test."

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4. Do you know what white box testing is?

Interviewers may ask this to further check whether you have experience with the different tests and processes involved in unit testing. In your answer, you can briefly describe where you've used white box testing.

Example: "Yes, I'm familiar with white box testing. It's a test we apply to software to check the structure, design and coding so that we can make improvements in the design, determine the usability of the application and verify its security. In my previous experience, it was the preferred test method for software, so I'm quite familiar with how to use it."

5. Do you know of any frameworks used in unit testing? Which ones have you used?

Here, the interviewers are looking to see if you know any coding language frameworks related to unit testing. You can answer by mentioning what you've worked with.

Example: "Yes, I know the different frameworks related to unit testing. I am particularly proficient in python and javascript unit tests because I have worked with them for most of my career as a developer."

6. What is the appropriate percentage of code coverage for unit testing? How did you settle on this percentage?

The interviewing panel may pose this question to you to assess your thought process and how you apply best practices while running unit tests. In your response, include specific technical details to show your expertise.

Example: "The percentage of code coverage I choose varies based on the particular software I'm testing. I mostly aim to use an average percentage of 80% for unit tests with new application code. I use this percentage as aiming higher can be costly and time-consuming and it may not provide any benefits in the end."

7. Can you differentiate between unit, integration, smoke and regression tests?

Here, the interviewers want to know if you know about other software tests apart from unit testing alone. You can respond by outlining what each test is.

Example: "These tests are usually used sequentially for testing various aspects of software under development. Unit tests isolate the smallest parts of an application and test them independently. Integration tests combine individual modules of software and test them as a group to examine the compliance of a system.

You then perform smoke tests to check whether systems are functioning properly and to ensure they don't stop working while they're in use. Regression testing is usually done last, and it's mostly performed after you've fixed a bug in your software and you want to make sure the new code you've applied is functioning properly."

8. Tell us about a challenge you have faced with unit testing and how you resolved it.

As an interviewee, you can look at this question in two ways, first from a technical aspect where you can describe an experience where a unit test for a particular software application posed some difficulty and second, from a character aspect where you can talk about how you handle challenges professionally and calmly using your problem-solving skills.

Example: "I experienced a challenge with unit testing one time in my previous place of work where the software kept displaying dependencies no matter how many times I ran unit tests for the individual parts of the application. It was slightly frustrating because I was under pressure to meet an urgent deadline, so I spoke with my supervisor and requested some extra time because I didn't want to submit an application that had errors.

After a while, I discovered that the code used to set up the application had an error and once I fixed it, the unit tests displayed no dependencies and I was able to apply the next process of development."

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