Conducting Technical Interviews for Web Developers
Presumably most readers of this blog will have been through a technical interview, and I'm betting that some of them have to give technical interviews as well. The purpose of this article is to go over some of the points to consider while conducting the interview, as well as suggestions for coming up with good questions to ask.
The first thing to understand as an interviewer is the requirements of the job position and what competencies a candidate must possess to be a good fit. If the job is for back-end development, then experience working with databases, performance tuning, and writing scalable code is crucial, and only a cursory understanding of HTML and CSS is required. Alternately, for UI development work, it isn't necessary to have a candidate who is wrapped up in the back-end code or has Big O notation at the tip of their tongue. There are certainly jacks-of-all-trades but when interviewing, it's better to focus on a few crucial areas than try to set high standards for all areas across the board.
These are some of the areas you may want to explore, depending on the requirements of the position:
Software Development Skills
- Proficiency in at least one high-level programming language: C++, Java
- Basic understanding of object-oriented design & analysis: classes/inheritance, interface vs implementation, encapsulation
- Experience with source code version control: SVN, CVS, P4
- Algorithms: binary search, quicksort
- Data structures: array, hash, linked list
- Complexity/performance analysis: Big O notation, performance testing
Web Development Skills
- Candidate tests web-deployed code in big 3 browsers: IE6+, Firefox 2+, Safari 2+
- Candidate uses browser-specific debugging tools: IE Web Developer Toolbar, Script Debugger/Script Editor (IE), Firebug (Firefox), WebKit/Drosera (Safari)
- Extensive knowledge of at least one JS framework: jQuery, Prototype, MooTools, YUI
- Experience creating dynamic, database-driven websites: PHP/JSP/ASP, MySQL etc
- Candidate uses Selenium for front-end automated testing
- Proficient in Fireworks or other image editing software, and knowledgeable about how to best export images for the web (CSS sprites, GIF/JPG/PNG, etc)
- Follows front-end best practices like CSS-based layouts and unobtrusive JavaScript
- Experience developing rich user interfaces (drag-and-drop, Ajax, web applications/RIAs, animation effects)
General and Miscellaneous Skills
- Familiarity with agile development methodology: scrum, burndown charts
- Proficiency at Search Engine Optimization (SEO), Search Engine Marketing (SEM), and related fields
- Knowledgeable about security vulnerabilities in code. Experience with threat analysis software (e.g. Fortify for Java)
- Familiarity with automated unit/integration tests: JUnit (Java), Selenium (front end)
- DBA-related skills such as database optimization
- Usability skills and knowledge of user behavior
- Project management skills
You can choose from these or other criteria to create a baseline for what makes an ideal candidate, ignoring the skills which aren't relevant. Once you've determined the skills necessary for the position, you may find that a Ph.D. is less qualified than someone without a college degree, depending on the skills required. So, never make assumptions about candidates simply based on their credentials or previous work experience. Developers can take drastically different amounts of time to complete tasks based on their prior experience and their ability to learn new things quickly.
Criteria for Various Job Positions
Let's look at a few example job positions and apply criteria from the list above.
Quality Assurance Engineer
An ideal QA engineer for a website will be an expert at creating automated test suites for front and back end, and should also be aware of security issues. So you could focus the interview on how they choose to write test suites, and come up with sample test suites for real-life objects (e.g. an elevator). Then you could ask them to list some of the common security threats (e.g. XSS, XSRF) and how to mitigate them.
Of course, if you already have a security engineer at your company, then you may not require your QA to wear another hat. But for most small- to medium-sized businesses, it's most efficient and cost effective for your developers to wear many hats. In fact, you may have a software developer who also does QA on the site, or a QA engineer who does project management. It all depends on the size of your company and the particular restraints in budget for your development team.
Software Developer / Programmer
Let me say right off the bat that I've never conducted a software development interview. But, I have been through a few, and I have an idea of what types of questions I would ask depending on the position.
There are some general software development skills that I would consider, such as experience with SVN (or other source control), automated testing, knowledge of data structures and algorithms, and perhaps object oriented design and analysis.
If the candidate is a Java developer, then you can ask about things like exception handling, reflection, familiarity with libraries (Swing, Struts, JSF etc), or other Java-specific questions. If they are a C++ developer then you should drill down into memory management, constructors/destructors, and other tropes of C++. It's up to you as the interviewer to determine the right questions to ask based on the job requirements, or to find someone else to conduct the interview if you can't. I know I would have a hard time conducting an interview with a C++ developer, unless I had a "cheatsheet" with all the correct answers to the questions.
Web Developer
For a web development position, the focus will be less on knowledge of traditional computer science and more on the candidate's knowledge of current best practices. Since best practices are always changing, the ideal web developer is enthusiastic and passionate about their field, and stays up to date by reading/contributing to blogs, belonging to email lists or other online communities and generally keeping "in the know" about the best way to solve problems.
Unlike traditional computer programming, in which code is executed in a controlled environment and changes to the language take years to complete, web development is a fast-paced field where changes happen all the time and code is executed in a wide range of environments, each with its own caveats and bugs. So, the ideal web developer is familiar with the issues surrounding development on each platform and environment, especially the Big 3 browsers (IE, Firefox and Safari).
Interview Coding Challenges
A great way to test a developer is to actually have them write some code for you. As they say, "the proof is in the pudding," and even a developer who successfully answers your questions may not be able to complete the test put forth. The test can be verbal (if conducting the interview by phone), on a whiteboard (if in person) or you can give the candidate a time limit and have them email you the code. The choice is yours, just make sure that you choose one modality for all candidates. It isn't fair comparing one candidate's typed code to what another said on the phone, or drew on a whiteboard. Also remember that generally typed code will be higher quality than that spoken over the phone or written on a whiteboard.
I've posted a few JavaScript challenges on this blog that would make good interview questions for a JS developer. You could also adapt some of these to other languages such as C++, PHP or Java:
Other challenges include reversing a string, reversing the words in a string, finding words that occur in both files, converting a clock from digital time to degrees (e.g. for an analog clock's hands), or generally any other relatively simple test you can come up with.
Layouts and HTML/CSS
For candidates doing CSS, just send them a mock-up and give them a reasonable amount of time to complete the HTML/CSS for it. You could optionally send a PSD or layered PNG, but more commonly people just send flat images. Really, the test shouldn't be much -- maybe the wireframe for a site, or a navbar, menu or UI component. You can also give them some challenge questions about explaining various CSS properties and how to use them. Challenges I've had in the past are:
- Make a banner and horizontal navbar that matches the dimensions of a wireframe
- Make a 3-column site with a fluid-width center column
- Create a vertical drop-down menu or horizontal flyout menu (bonus points for a cross-browser CSS-only solution, though JavaScript solutions are accepted)
- Explain the difference between absolute and relative positioning, and what happens when you put one type of element inside another
- Explain the difference in block and inline display
- Explain what effect float has on elements. Does float change elements' display property, and if so, does it make it block or inline? (Answer: block)
- Explain what effect z-index has on elements and the natural z-index order.
- Explain CSS specificity.
On this last point, CSS specificity, I find it's fun to show a list of CSS rules and ask which rule wins out. If you know the trick how rules are calculated, it's easy to tell:
<!-- Sample HTML code -->
<p class="description" id="main">
Some text.
<span class="special">
Some more text.
</span>
</p>
/* Challenge:
* What color is the "special" text? */
body .description span {
color: blue;
}
p.description {
color: red;
}
body .description .special {
color: pink;
}
#main .special {
color: orange;
}
#main span {
color: green;
}
For those of you unfamiliar with how CSS specificity is calculated by browsers, it is a point-based system where the selector with the highest point value wins. The three main things selectors consist of are elements, classes and ids. These are worth 1 point, 10 points and 100 points respectively. Given this knowledge, do you know which rule wins out in the above example?
By calculating the point values of the rules, you see that the rule #main .special is worth 110 points and is the winner, hence the color of the "special" text is orange.
Hiring People who Write Good Code
Besides "just working", the criteria for what makes good code differs from person to person, but in general, you'll want to look for readability, useful comments, simplicity, accuracy, good formatting and code that doesn't repeat itself. So, even if two candidates submit working solutions to a problem, you can still determine which is the better candidate by analyzing the code itself with these criteria in mind.
In addition to this, you may consider their talent, as opposed to their current skill in any one area. If the candidate is highly talented and passionate about the position, then they will most likely be able to learn any particular skills required for the position, within reason. Of course, talented individuals should demonstrate expertise in a number of domains already, which will give you an idea of their potential for growth in job-specific areas. You may also consider their participation in online forums, discussion groups and blogs (like this one, wink wink) related to their job field. Active participation in online communities demonstrates dedication and passion for those particular fields of interest.
I hope this article has been informative for interviewers and perhaps interviewees as well. Feel free to post your favorite (or least favorite) interview questions.
Presumably most readers of this blog will have been through a technical interview, and I'm betting that some of them have to give technical interviews as well. The purpose of this article is to go over some of the points to consider while conducting the interview, as well as suggestions for coming up with good questions to ask.
The first thing to understand as an interviewer is the requirements of the job position and what competencies a candidate must possess to be a good fit. If the job is for back-end development, then experience working with databases, performance tuning, and writing scalable code is crucial, and only a cursory understanding of HTML and CSS is required. Alternately, for UI development work, it isn't necessary to have a candidate who is wrapped up in the back-end code or has Big O notation at the tip of their tongue. There are certainly jacks-of-all-trades but when interviewing, it's better to focus on a few crucial areas than try to set high standards for all areas across the board.
These are some of the areas you may want to explore, depending on the requirements of the position:
Software Development Skills
- Proficiency in at least one high-level programming language: C++, Java
- Basic understanding of object-oriented design & analysis: classes/inheritance, interface vs implementation, encapsulation
- Experience with source code version control: SVN, CVS, P4
- Algorithms: binary search, quicksort
- Data structures: array, hash, linked list
- Complexity/performance analysis: Big O notation, performance testing
Web Development Skills
- Candidate tests web-deployed code in big 3 browsers: IE6+, Firefox 2+, Safari 2+
- Candidate uses browser-specific debugging tools: IE Web Developer Toolbar, Script Debugger/Script Editor (IE), Firebug (Firefox), WebKit/Drosera (Safari)
- Extensive knowledge of at least one JS framework: jQuery, Prototype, MooTools, YUI
- Experience creating dynamic, database-driven websites: PHP/JSP/ASP, MySQL etc
- Candidate uses Selenium for front-end automated testing
- Proficient in Fireworks or other image editing software, and knowledgeable about how to best export images for the web (CSS sprites, GIF/JPG/PNG, etc)
- Follows front-end best practices like CSS-based layouts and unobtrusive JavaScript
- Experience developing rich user interfaces (drag-and-drop, Ajax, web applications/RIAs, animation effects)
General and Miscellaneous Skills
- Familiarity with agile development methodology: scrum, burndown charts
- Proficiency at Search Engine Optimization (SEO), Search Engine Marketing (SEM), and related fields
- Knowledgeable about security vulnerabilities in code. Experience with threat analysis software (e.g. Fortify for Java)
- Familiarity with automated unit/integration tests: JUnit (Java), Selenium (front end)
- DBA-related skills such as database optimization
- Usability skills and knowledge of user behavior
- Project management skills
You can choose from these or other criteria to create a baseline for what makes an ideal candidate, ignoring the skills which aren't relevant. Once you've determined the skills necessary for the position, you may find that a Ph.D. is less qualified than someone without a college degree, depending on the skills required. So, never make assumptions about candidates simply based on their credentials or previous work experience. Developers can take drastically different amounts of time to complete tasks based on their prior experience and their ability to learn new things quickly.
Criteria for Various Job Positions
Let's look at a few example job positions and apply criteria from the list above.
Quality Assurance Engineer
An ideal QA engineer for a website will be an expert at creating automated test suites for front and back end, and should also be aware of security issues. So you could focus the interview on how they choose to write test suites, and come up with sample test suites for real-life objects (e.g. an elevator). Then you could ask them to list some of the common security threats (e.g. XSS, XSRF) and how to mitigate them.
Of course, if you already have a security engineer at your company, then you may not require your QA to wear another hat. But for most small- to medium-sized businesses, it's most efficient and cost effective for your developers to wear many hats. In fact, you may have a software developer who also does QA on the site, or a QA engineer who does project management. It all depends on the size of your company and the particular restraints in budget for your development team.
Software Developer / Programmer
Let me say right off the bat that I've never conducted a software development interview. But, I have been through a few, and I have an idea of what types of questions I would ask depending on the position.
There are some general software development skills that I would consider, such as experience with SVN (or other source control), automated testing, knowledge of data structures and algorithms, and perhaps object oriented design and analysis.
If the candidate is a Java developer, then you can ask about things like exception handling, reflection, familiarity with libraries (Swing, Struts, JSF etc), or other Java-specific questions. If they are a C++ developer then you should drill down into memory management, constructors/destructors, and other tropes of C++. It's up to you as the interviewer to determine the right questions to ask based on the job requirements, or to find someone else to conduct the interview if you can't. I know I would have a hard time conducting an interview with a C++ developer, unless I had a "cheatsheet" with all the correct answers to the questions.
Web Developer
For a web development position, the focus will be less on knowledge of traditional computer science and more on the candidate's knowledge of current best practices. Since best practices are always changing, the ideal web developer is enthusiastic and passionate about their field, and stays up to date by reading/contributing to blogs, belonging to email lists or other online communities and generally keeping "in the know" about the best way to solve problems.
Unlike traditional computer programming, in which code is executed in a controlled environment and changes to the language take years to complete, web development is a fast-paced field where changes happen all the time and code is executed in a wide range of environments, each with its own caveats and bugs. So, the ideal web developer is familiar with the issues surrounding development on each platform and environment, especially the Big 3 browsers (IE, Firefox and Safari).
Interview Coding Challenges
A great way to test a developer is to actually have them write some code for you. As they say, "the proof is in the pudding," and even a developer who successfully answers your questions may not be able to complete the test put forth. The test can be verbal (if conducting the interview by phone), on a whiteboard (if in person) or you can give the candidate a time limit and have them email you the code. The choice is yours, just make sure that you choose one modality for all candidates. It isn't fair comparing one candidate's typed code to what another said on the phone, or drew on a whiteboard. Also remember that generally typed code will be higher quality than that spoken over the phone or written on a whiteboard.
I've posted a few JavaScript challenges on this blog that would make good interview questions for a JS developer. You could also adapt some of these to other languages such as C++, PHP or Java:
Other challenges include reversing a string, reversing the words in a string, finding words that occur in both files, converting a clock from digital time to degrees (e.g. for an analog clock's hands), or generally any other relatively simple test you can come up with.
Layouts and HTML/CSS
For candidates doing CSS, just send them a mock-up and give them a reasonable amount of time to complete the HTML/CSS for it. You could optionally send a PSD or layered PNG, but more commonly people just send flat images. Really, the test shouldn't be much -- maybe the wireframe for a site, or a navbar, menu or UI component. You can also give them some challenge questions about explaining various CSS properties and how to use them. Challenges I've had in the past are:
- Make a banner and horizontal navbar that matches the dimensions of a wireframe
- Make a 3-column site with a fluid-width center column
- Create a vertical drop-down menu or horizontal flyout menu (bonus points for a cross-browser CSS-only solution, though JavaScript solutions are accepted)
- Explain the difference between absolute and relative positioning, and what happens when you put one type of element inside another
- Explain the difference in block and inline display
- Explain what effect float has on elements. Does float change elements' display property, and if so, does it make it block or inline? (Answer: block)
- Explain what effect z-index has on elements and the natural z-index order.
- Explain CSS specificity.
On this last point, CSS specificity, I find it's fun to show a list of CSS rules and ask which rule wins out. If you know the trick how rules are calculated, it's easy to tell:
<!-- Sample HTML code --> <p class="description" id="main"> Some text. <span class="special"> Some more text. </span> </p>
/* Challenge:
* What color is the "special" text? */
body .description span {
color: blue;
}
p.description {
color: red;
}
body .description .special {
color: pink;
}
#main .special {
color: orange;
}
#main span {
color: green;
}
For those of you unfamiliar with how CSS specificity is calculated by browsers, it is a point-based system where the selector with the highest point value wins. The three main things selectors consist of are elements, classes and ids. These are worth 1 point, 10 points and 100 points respectively. Given this knowledge, do you know which rule wins out in the above example?
By calculating the point values of the rules, you see that the rule #main .special is worth 110 points and is the winner, hence the color of the "special" text is orange.
Hiring People who Write Good Code
Besides "just working", the criteria for what makes good code differs from person to person, but in general, you'll want to look for readability, useful comments, simplicity, accuracy, good formatting and code that doesn't repeat itself. So, even if two candidates submit working solutions to a problem, you can still determine which is the better candidate by analyzing the code itself with these criteria in mind.
In addition to this, you may consider their talent, as opposed to their current skill in any one area. If the candidate is highly talented and passionate about the position, then they will most likely be able to learn any particular skills required for the position, within reason. Of course, talented individuals should demonstrate expertise in a number of domains already, which will give you an idea of their potential for growth in job-specific areas. You may also consider their participation in online forums, discussion groups and blogs (like this one, wink wink) related to their job field. Active participation in online communities demonstrates dedication and passion for those particular fields of interest.
I hope this article has been informative for interviewers and perhaps interviewees as well. Feel free to post your favorite (or least favorite) interview questions.
posted by Jonah Dempcy at
2:41 PM
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