I started working with JavaScript around 2006. At that time JS was used as a lightweight scripting language for the Web. There was no React/Vue/Svelte and so on, so programmers around the world used pure JavaScript aka vanilla JavaScript as it is called today.  But over time the language was developing and so was the ecosystem around it. I like modern JavaScript because it is a relatively small language, despite years of evolution. So one can quickly learn it. Another, practical advantage is a very strong, optimizing virtual machines. You may hear a lot of criticism of the language but half of the problems are easily solved by a good discipline of a programmer, which is required in any case and in any language.  

I recommend everyone to study the basics of compiler construction, interpretation, or language virtual machines to better understand why our technologies are built the way they are, what are limitations of different approaches, and what are motivations of various design decisions in the programming languages we use every day. I'm myself an author of several experimental programming languages and have developed several processors and parsers for existing languages.

I first started to seriously use C when I became involved in embedded development. C is a compact and very powerful language. I recommend everyone to study it at least to better understand how computer systems work at lower levels. Also, C allows one to write highly optimized programs. So I personally used C for (sub)projects where speed was an important factor, for example in my data compression experiments, or in my university project about software ray-tracing. Also C I implemented several embedded projects in C and once even wrote a Windows driver in it.

2D and 3D computer graphics is the thing that fascinated me for my whole life. It's one of the reasons I became a programmer. 
I'm especially interested in software rendering, although I've also implemented a 3d physics engine and some other engine-related code.
In the field of software rendering, I always was a believer in the ray-tracing approach as a future-proof way to go. Today it started to be supported at the hardware level. But hardware makes you a little bit more constrained in your approaches.
So, although I have some experience with OpenGL and Direct3D, my own fully software ray tracer was much more interesting for me to create from scratch. This reimplementation of a known-how-to-do thing is important in my opinion to gather the full understanding of what's going on.

Assembly is a language of a computer. So studying assembly gives one a deep understanding of computer microarchitecture. I have used Assembly for different parts of different projects writing critical functions in it for speed. It is the way Assembly is used today too. We write the most critical code in it or we write in it the code if it can not be written in any other language - this happened in operating systems development or in embedded programming, for example. Another cool project I've once written is an on-the-fly translator of mathematical notation formula to a sequence of assembly instructions which the program then could execute instead of reinterpretation the formula every time. This gave 10x-100x speedup for the program. In essence, this was a small specialized JIT compiler for mathematical expressions. I couldn't have done it without my knowledge of assembly language. 

RLE, statistics, Huffman encoding, arithmetic/entropy encoding, dictionary/LZ compression, n-order models, PPM, DMC, PAQ, BPE, word vectors, lossless compression using neural networks, online learning, JPEG format, images optimization, source-to-source minification.

I used Python as a cross-platform language to build desktop applications, to write various scripts, and lately to do tutoring and experiments with machine learning.

I'm a self-taught Pascal programmer. It was my main language of use in my school days when I was learning programming myself. I wrote hundreds of tiny projects in Object Pascal including my own Pascal parser) Also, I wrote a book on the Pascal language but now I believe it has no value for anyone except me)
What I like about this language it's that is more or less semantically compact, more or less easy to use, and powerful. For example, Object Pascal it has built-in dynamic arrays, strings, some RTTI, inline assembly, and a fast, very fast compiler.)
Delphi RAD IDE had a fast and easy-to-use GUI designer - it was possible to create useful UX within minutes!)

I teach the basics of Java and the basics of computer science and programming using Java. I rarely use Java myself, because usually I prefer C# for non-mobile development or Kotlin for mobile development. However even so I still believe that one should know Java to be really productive in the Android ecosystem.

I've used C# both commercially at work and in my own side projects. This programming language combines powerful primitives and good performance. So it can be used in a Python-ish way for experiments without the need to manually control memory, with a more or less fast compilation speed. Also, C# is, of course, a good fit for Windows Desktop application development because the VS IDE is good and it has a handy visual GUI editor.

C/C++ is an essential technology to know for many senior programmers because these languages form a basis for our industry. Personally, I believe that C++ is overengineered language. So I use it with great caution and try to avoid all the complications it has.
I frequently used it just as a more advanced C, with overloading, templates, and so on.
But one also can use it as a very different language, more high-level, abstract, and slow to compile :) - as of 2023 at least, the problem is slowly being solved. 

I used Kotlin instead of Java to build an Android application recently. It was an application related to real-time low-latency audio communication. (The application used UDP network protocol to achieve low-latency requirements.)
I like Kotlin better than Java because it's much less verbose. 

I have little experience with the Go language, to be honest. But it's straightforward to learn. One can use it almost right away. Especially if you know its philosophy and the C language. It is easier than C because it is higher level, for example, it has a garbage collector. Moreover, the ecosystem around the language, especially the package manager, makes the life of a Go developer considerably easier too, at least at the start of a journey. I used Go for some back-end work and in the development of my own code editor.

Lua is a small, but powerful dynamic language. It is very similar to JavaScript in its early days (around EcmaScript 3/4). Yes, they have different syntax. But the syntax is only a surface level of a language, inside the two are really similar. 
What truly sets Lua apart is its lightweight yet high-performance virtual machine, making it an ideal choice as a scripting language for projects developed in C/C++ and the like. 
I myself am a fan of a code editor called TextAdept which is a minimalist hackable code editor. It is programmed using Lua and is very configurable. This project was my main reason to learn more about Lua. 
Today I teach Lua to other people and find it a good language, on par with JavaScript / TypeScript for tasks on basics of programming and algorithmic thinking. Though I would recommend learning some compiled language too, to have a better understanding of what's going on under the hood. 

Visual Basic and Basic itself were my first languages. I still remember how easy it was to create some small GUI applications with the help of Visual Basic GUI designer. Later I moved to Delphi (Object Pascal), because it compiled to native code and had more features including for example inline Assembly, pointers, and so on.

I've used PHP when it was a relatively new language. At that time the so-called LAMP stack was very popular and the default choice for many projects. I quickly learn the language and ecosystem granted it is rather simple. I built one system used later in our college for exams with PHP/MySQL, and some time after I also learned WordPress CMS and used it to create multiple commercial small-sized projects. But mostly I try to avoid PHP even though the language and ecosystem around it greatly changed, the language still just doesn't feel right to my taste. So I try to avoid it as much as possible. 

I used Ruby for small automation scripts instead of Bash/Python and also I implemented one commercial Web project in Ruby + HTML/CSS/JS using the Ruby On Rails framework.
Also, I wrote my personal blog site in pure Ruby, again because it is convenient to write small scripts in it. And these scripts don't take long to start as it is with JS or TS, so one even can use Ruby as a stateless CGI language at the back end. 
The language and the framework had some interesting features at the time they were popular. One thing that was well developed here was the principle of convention over configuration. It is an interesting principle making some tasks easier but some other tasks harder. 

Personal details

Ruslan O.

Summary

Technical skills