The 10 Most Common Mistakes Web Developers Make: A Tutorial for Developers

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Since the term the World Wide Web was coined back in 1990, web application development has evolved from serving static HTML pages to completely dynamic, complex business applications.

Today we have thousands of digital and printed resources that provide step-by-step instructions about developing all kinds of different web applications. Development environments are “smart” enough to catch and fix many mistakes that early developers battled with regularly. There are even many different development platforms that easily turn simple static HTML pages into highly interactive applications.

All of these development patterns, practices, and platforms share common ground, and they are all prone to similar web development issues caused by the very nature of web applications.

The purpose of these web development tips is to shed light on some of the common mistakes made in different stages of the web development process and to help you become a better developer. I have touched on a few general topics that are common to virtually all web developers such as validation, security, scalability, and SEO. You should of course not be bound by the specific examples I’ve described in this guide, as they are listed only to give you an idea of the potential problems you might encounter.

Think of all the web development mistakes that could be avoided at this campout!

Common mistake #1: Incomplete input validation

Validating user input on client and server side is simply a must do! We are all aware of the sage advice “do not trust user input” but, nevertheless, mistakes stemming from validation happen all too often.

One of the most common consequences of this mistake is SQL Injection which is in OWASP Top 10 year after year.

Remember that most front-end development frameworks provide out-of-the-box validation rules that are incredibly simple to use. Additionally, most major back-end development platforms use simple annotations to assure that submitted data are adhering to expected rules. Implementing validation might be time consuming, but it should be part of your standard coding practice and never set aside.

Common mistake #2: Authentication without proper Authorization

Before we proceed, let’s make sure we are aligned on these two terms. As stated in the 10 Most Common Web Security Vulnerabilities:

Authentication: Verifying that a person is (or at least appears to be) a specific user, since he/she has correctly provided their security credentials (password, answers to security questions, fingerprint scan, etc.).

Authorization: Confirming that a particular user has access to a specific resource or is granted permission to perform a particular action.

Stated another way, authentication is knowing who an entity is, while authorization is knowing what a given entity can do.

Let me demonstrate this issue with an example:

Consider that your browser holds currently logged user information in an object similar to the following:


When doing a password change, your application makes the POST:

POST /changepassword/:username/:newpassword

In your /changepassword method, you verify that user is logged and token has not expired. Then you find the user profile based on the :username parameter, and you change your user’s password.

So, you validated that your user is properly logged-in, and then you executed his request thus changing his password. Process seems OK, right? Unfortunately, the answer is NO!

At this point it is important to verify that the user executing the action and the user whose password is changed are the same. Any information stored on the browser can be tampered with, and any advanced user could easily update username:'elvis' to username:'Administrator' without using anything else but built-in browser tools.

So in this case, we just took care of Authentication making sure that the user provided security credentials. We can even add validation that /changepassword method can only be executed by Authenticated users. However, this is still not enough to protect your users from malicious attempts.

You need to make sure that you verify actual requestor and content of request within your /changepassword method and implement proper Authorization of the request making sure that user can change only her data.

Authentication and Authorization are two sides of the same coin. Never treat them separately.

Common mistake #3: Not ready to scale

In today’s world of high speed development, startup accelerators, and instant global reach of great ideas, having your MVP (Minimum Viable Product) out in the market as soon as possible is a common goal for many companies.

However, this constant time pressure is causing even good web development teams to often overlook certain issues. Scaling is often one of those things teams take for granted. The MVP concept is great, but push it too far, and you’ll have serious problems. Unfortunately, selecting a scalable database and web server and separating all application layers on independent scalable servers is not enough. There are many details you need to think about if you wish to avoid rewriting significant parts of your application later - which becomes a major web development problem.

For example, say that you choose to store uploaded profile pictures of your users directly on a web server. This is a perfectly valid solution–files are quickly accessible to the application, file handling methods are available in every development platform, and you can even serve these images as static content, which means minimum load on your application.

But what happens when your application grows, and you need to use two or more web servers behind a load balancer? Even though you nicely scaled your database storage, session state servers, and web servers, your application scalability fails because of a simple thing like profile images. Thus, you need to implement some kind of file synchronization service (that will have a delay and will cause temporary 404 errors) or another workaround to assure that files are spread across your web servers.

This bloated blowfish is full of web development tips and tutorials to avoid common mistakes.

What you needed to do to avoid the problem in the first place was just use shared file storage location, database, or any other remote storage solution. It would have probably cost few extra hours of work to have it all implemented, but it would have been worth the trouble.

Common mistake #4: Wrong or missing SEO

The root cause of incorrect or missing SEO best practices on web sites is misinformed “SEO specialists”. Many web developers believe that they know enough about SEO and that it is not especially complex, but that’s just not true. SEO mastery requires significant time spent researching best practices and the ever-changing rules about how Google, Bing, and Yahoo index the web. Unless you constantly experiment and have accurate tracking + analysis, you are not a SEO specialist, and you should not claim to be one.

Furthermore, SEO is too often postponed as some activity that is done at the end. This comes at a high price of web development issues. SEO is not just related to setting good content, tags, keywords, meta-data, image alt tags, site map, etc. It also includes eliminating duplicate content, having crawlable site architecture, efficient load times, intelligent back linking, etc.

Like with scalability, you should think about SEO from the moment you start building your web application, or you might find that completing your SEO implementation project means rewriting your whole system.

Common mistake #5: Time or processor consuming actions in request handlers

One of the best examples of this mistake is sending email based on a user action. Too often developers think that making a SMTP call and sending a message directly from user request handler is the solution.

Let’s say you created an online book store, and you expect to start with a few hundred orders daily. As part of your order intake process, you send confirmation emails each time a user posts an order. This will work without problem at first, but what happens when you scale your system, and you suddenly get thousands of requests sending confirmation emails? You either get SMTP connection timeouts, quota exceeded, or your application response time degrades significantly as it is now handling emails instead of users.

Any time or processor consuming action should be handled by an external process while you release your HTTP requests as soon as possible. In this case, you should have an external mailing service that is picking up orders and sending notifications.

Common mistake #6: Not optimizing bandwidth usage

Most development and testing takes place in a local network environment. So when you are downloading 5 background images each being 3MB or more, you might not identify an issue with 1Gbit connection speed in your development environment. But when your users start loading a 15MB home page over 3G connections on their smartphones, you should prepare yourself for a list of complaintsand problems.

Optimizing your bandwidth usage could give you a great performance boost, and to gain this boost you probably only need a couple of tricks. There are few things that many good web deveopers do by default, including:

  1. Minification of all JavaScript
  2. Minification of all CSS
  3. Server side HTTP compression
  4. Optimization of image size and resolution

Common mistake #7: Not developing for different screen sizes

Responsive design has been a big topic in the past few years. Expansion of smartphones with different screen resolutions has brought many new ways of accessing online content, which also comes with a host of web development issues. The number of website visits that come from smartphones and tablets grows every day, and this trend is accelerating.

In order to ensure seamless navigation and access to website content, you must enable users to access it from all types of devices.

There are numerous patterns and practices for building responsive web applications. Each development platform has its own tips and tricks, but there are some frameworks that are platform independent. The most popular is probably Twitter Bootstrap. It is an open-source and free HTML, CSS, and JavaScript framework that has been adopted by every major development platform. Just adhere to Bootstrap patterns and practices when building your application, and you will get responsive web application with no trouble at all.

Common mistake #8: Cross browser incompatibility

The development process is, in most cases, under a heavy time pressure. Every application needs to be released as soon as possible and even good web developers are often focused on delivering functionality over design. Regardless of the fact that most developers have Chrome, Firefox, IE installed, they are using only one of these 90% of the time. It is common practice to use one browser during development and just as the application nears completion will you start testing it in other browsers. This is perfectly reasonable–assuming you have a lot of time to test and fix issues that show up at this stage.

However, there are some web development tips that can save you significant time when your application reaches the cross-browser testing phase:

  1. You don’t need to test in all browsers during development; it is time consuming and ineffective. However, that does not mean that you cannot switch browsers frequently. Use a different browser every couple of days, and you will at least recognize major problems early in development phase.
  2. Be careful of using statistics to justify not supporting a browser. There are many organizations that are slow in adopting new software or upgrading. Thousands of users working there might still need access to your application, and they cannot install the latest free browser due to internal security and business policies.
  3. Avoid browser specific code. In most cases there is an elegant solution that is cross-browser compatible.

Common mistake #9: Not planning for portability

Assumption is the mother of all problems! When it comes to portability, this saying is more true than ever. How many times have you seen issues in web development like hard coded file paths, database connection strings, or assumptions that a certain library will be available on the server? Assuming that the production environment will match your local development computer is simply wrong.

Ideal application setup should be maintenance-free:

  1. Make sure that your application can scale and run on a load-balanced multiple server environment.
  2. Allow simple and clear configuration–possibly in a single configuration file.
  3. Handle exceptions when web server configuration is not as expected.

Common mistake #10: RESTful anti patterns

RESTful API’s have taken their place in web development and are here to stay. Almost every web application has implemented some kind of REST services, whether for internal use or integrating with external system. But we still see broken RESTful patterns and services that do not adhere to expected practices.

Two of the most common mistakes made when writing a RESTful API are:

  1. Using wrong HTTP verbs. For example using GET for writing data. HTTP GET has been designed to be idempotent and safe, meaning that no matter how many times you call GET on the same resource, the response should always be the same and no change in application state should occur.
  2. Not sending correct HTTP status codes. The best example of this mistake is sending error messages with response code 200.

     HTTP 200 OK
         message:'there was an error'

You should only send HTTP 200 OK when the request has not generated an error. In the case of an error, you should send 400, 401, 500 or any other status code that is appropriate for the error that has occurred.

A detailed overview of standard HTTP status codes can be found here.

Wrap up

Web development is an extremely broad term that can legitimately encompass development of a website, web service, or complex web application.

The main takeaway of this web development guide is the reminder that you should always be careful about authentication and authorization, plan for scalability, and never hastily assume anything - or be ready to deal with a long list of web development problems!

About the author

Demir Selmanovic, Bosnia and Herzegovina
member since May 23, 2014
Demir is a developer and project manager with over 15 years of professional experience in a wide range of software development roles. He excels as a solo developer, team member, team leader, or manager of multiple distributed teams. He works closely with clients to define ideas and deliver products. [click to continue...]
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André Lourenço
I agree to pretty much everything said here, except for the RESTful anti patterns item 2. There are a lot of discussion about whether you should use the wide range of HTTP status codes available in HTTP or you should make your own application pattern. Personally I like the latter approach because it keeps your API guide simple, you just have to define what'
Ismail Akram
That makes hard to develop too.. also if new developer comes in he would need to learn a lot. if follow the standard practices its great for maintainability.
Demir Selmanovic
I understand the discussion and points made for using custom patterns. But I don't think that two are mutually exclusive. I think that these mostly come from wrong assumption that HTTP Status Code should be some kind of Message Code generated by the application, which is, in my view, wrong assumption. Also, most of front end frameworks expect standard HTTP codes in order to be able to call proper callbacks (success, error). I think that best approach is to have combination of proper HTTP Status Codes and custom Message Codes like: HTTP Status 401 Unauthorized {Code: 1001, Message: "Token Expired"}, or HTTP Status 401 Unauthorized {Code: 1000, Message: "Token not provided"}. Standard HTTP Status Codes are far from sufficient for all the scenarios and we need to have some custom codes for our application, but these two should not be mixed.
Amazing list and I faced 2 of them quite a few times when I asked to complete the incomplete sites of others. In most of them there were no proper input validations used and missing SEO elements. Great stuff Demir and I enjoyed reading.
Thanks for this list, it covers a vast part of mistakes devs make on their way. Many of these mistakes actually intertwine with web design, which can be both a deterrent and a magnet for visitors. Here's our list of dev- and design oriented mistakes websites can suffer from:
These mistakes may not be noticed by those developers who are just new into their job. This technique that you share will surely give them enough information on what are they things that they need to avoid in order to have a wonderful output.
Atul Chaudhary
Hi Demir, Thanks for such a nice piece of information. As most of us are using Wordpress platform for creating sites, I would like to know more about WordPress security issues. Can you provide some inputs on the same. Thanks in advance. Atul
Great article. Really very common web development mistakes you elaborated here.While developing any website we should focus on these points too. Thanks for sharing these tips.
Aman Upadhyay
Hi, I agree to all the points really informative post and apt as well. However a related work on some very common website mistakes that we often ignore till the time they do visible damage are
Sudhir Gupta
its a nice article. found lot of information. thanks demir
Jake McNeil
I can’t begin to tell you how much I enjoy reading your blog. I always find some useful information even if it is a topic I am familiar with.
Natalia Mykytka
Great article i should say. Front end or client side web development is a complex job that requires both programming rigor and good eye for aesthetics. These guys take the back end structure and fuse it with the visual look created by the designer. Front end coders need to collaborate with clients, managers, content creators, designers and back enders, thus it’s next to impossible to avoid any problems with front end. We hope some of our ideas will come in handy and make your communication with the front end developer easier and more fruitful. Read more here:
Monica Johns
Great post!! Great way to represent here because mostly developers make these types of mistakes,here provide great solutions..
Paul Cook
Great article. Thanks for putting this together.Looking forward to more of this.I found an interesting article regarding website development here you can find it
Dieter Ziegler
Hey, thank you for sharing this article. It was indeed very helpful. I came across this brilliant article, you may be interested in reading.
Nice article, common mistakes, web developers can eliminate much frustration that others have already endured. Not only is it important to acknowledge, but when we understand the impact of a mistake and take measures to avoid it, we can create a development process catered to our preferences.
Rochell Marco
Thanks for such an informative, thought with us. Every web developer should consider these mistakes and avoid these mistakes which help to save their time. It should require them to learn from each mistake and make a perfect bug free app. Thanks for sharing!
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About the author
Demir Selmanovic
JavaScript Developer
Demir is a developer and project manager with over 15 years of professional experience in a wide range of software development roles. He excels as a solo developer, team member, team leader, or manager of multiple distributed teams. He works closely with clients to define ideas and deliver products.