JavaScript vs Scala
When comparing JavaScript vs Scala, the Slant community recommends JavaScript for most people. In the question“What is the best programming language to learn first?” JavaScript is ranked 9th while Scala is ranked 21st. The most important reason people chose JavaScript is:
If you run a web browser you already have JavaScript installed and can get started right away. Modern browsers such as Chrome also have very powerful programming consoles built into them. Aside from the browser console, you can also use online Javascript playgrounds such as JS Bin and JS Fiddle. Even from a tablet.
Specs
Question Ranking
Pros
Pro No installation required
If you run a web browser you already have JavaScript installed and can get started right away. Modern browsers such as Chrome also have very powerful programming consoles built into them.
Aside from the browser console, you can also use online Javascript playgrounds such as JS Bin and JS Fiddle. Even from a tablet.
Pro Required for web development
If you are looking to create web projects, you will have to learn Javascript in order to develop the client side code. If you learn the foundations of programming in JavaScript you can reapply that education later in building web applications.
Pro Runs on both the browser and the server
With Node.js, it is now possible to run JavaScript as a web server. This would allow you to be able to create server based applications sooner than would if you had to learn a separate programming language as well for server side code.
As JavaScript is the only language supported by web browsers it puts it in the unique situation of being the only programming language that's available on both the client side and server side.
Pro Instant gratification
While it's easy to argue that Python will give you instant gratification, JavaScript is far better in this regard. Make a small change to a page and it's immediately visible in the browser. You can throw in a JavaScript library like jQuery with minimal fuss.
Pro First-class functions with lexical closures
While certainly not the only language with these features, this pro alone is so powerful that it compensates for most of JavaScript's problems. You'll learn to use closures and higher-order functions well in JavaScript, because you have to. And then you can generalize this knowledge to any other language that has these, and the good ones do.
Pro Easy to build an application
By using the UI capabilities in HTML and CSS you can develop substantial applications with graphical interfaces more quickly and with less effort than in other languages which would require you to learn a windowing library.
Building a useful application is one of the best ways to learn a new language and because of the low learning curve for creating applications you can create more substatial programs and learn more practical programming priciple faster.
Pro Massive ecosystem
JavaScript has one of the largest programming ecosystems, as shown by the being the most popular language for projects on GitHub. As there are so many projects written in JavaScript there are lots of libraries available to build off of and many of them are written to be easy to use and integrate into other projects.
There are also lots of resources available for learning JavaScript. Other than traditional tutorials, language learning sites such as Codecademy have JavaScript courses. The Mozilla Developer Database also serves as a great resource for learning about the standard libraries built into JavaScript.
Pro C-like syntax
After learning Javascript, you will feel at home in other languages as C-like syntax is very common.
Pro Complete dev stack can be run online
With codepen.io and other prototyping tools, you can learn Javascript from a mobile device. You don't even need a computer. It can be learned from an internet cafe or public library.
Pro JSON is native to JS
JSON is arguably a "must-learn". With JS, that's one less additional syntax to learn.
Pro Very good debugger
Has a built in debugger with break points, watches that work on local values, and a console that you can use to edit anything at any time. Both in the browser (eg: Chrome), and server (eg: Nodejs).
Pro Can be very simple (teachable)
By setting a few ground-rules (effectively coding in a subset of JS), JS is one of the simplest languages to learn (requiring very few must-learn prerequisite concepts).
Pro Several Platforms to use the web stack and JS to create multi-platform apps
Opens the door to native application development as well as just websites. Use with React Native, Weex or Quasar (Vue), PhoneGap or Cordova, NativeScript... (etc) to build native apps. Use mostly the same code base for multi-platform and web.
Pro Great tools for development
Flow, JSHint/ESLint, Babel, npm, etc.
Pro Atwood's Law "Any application that can be written in JavaScript, will eventually be written in JavaScript."
May also be a con.
Pro Prototype based Object Oriented System
Being object oriented, it supports the predominate and powerful programming approach. Being prototype based, it provides an intuitive approach to OOP. Whereas other popular languages use classes, which focus on thinking in the abstract, Javascript's prototypes allow you to focus on thinking in the concrete.
For example, in a prototypical language, you think of a rectangle, and define it. You now have a rectangle. Let's say you want a red rectangle, you copy the rectangle and give it the property red. Want a blue one? Copy rectangle again give it a blue. Big blue rectangle? Copy blue rectangle and make it big. In a class-based language, first you describe a rectangle, describe a red rectangle as a type of rectangle, describe blue and big blue rectangles, and now that you have described them, you must create instances of them, and only then do you have the rectangles. Essentially: Classes are factories for objects, prototypes are objects themselves, and the latter method was created to be more intuitive, which is particularly advantageous to beginners.
Pro High demand for JavaScript developers
If you're looking for a career as a developer, JavaScript is the place to focus your attention. There is a huge demand for good developers especially in frameworks such as React and Angular.
Pro Modern ESNext is far better than the JS of days past
Modern JS has made great strides, and can be targerted to older (or non-standard) browsers using Babel. There are new language constructs that can make programming in JS comfortable.; e.g.: async / await ( <3 ).
Pro Integrates very well with UE4
Coding an immersive 3D game can retain the attention of new programmers. ncsoft/Unreal.js.
Pro Speed (most implementations)
JS/ES is in the running for the fastest interpreted language given the optimizations and JIT integration of popular implementations. On the other hand, it fails utterly when compared with compiled (to native or VM code) languages.
Pro Extremely popular
JavaScript usually tops the lists for most popular languages in use today and rightly so. It's used almost everywhere and it's in high demand, making it very easy to find a job for anyone who knows JavaScript. This helps make it desirable for a first language, as it will often be used in the future.
Pro The most used language in the whole Solar System in amount of scripts/applications
Because it runs in many different environments, it is the most used language in the world.
Pro Immutable values
The immutable values make it perfect for working with concurrency
Pro Multiparadigm
Scala supports both Functional and Object Oriented styles of programming. Beginners can learn both paradigms without having to learn a new language, and experts can switch between the two according to what best suits their needs at the time.
Pro Type inference
Scala offers type inference, which, while giving the same safety as Java's type system, allows programmers to focus on the code itself, rather than on updating type annotations.
Pro Compiles to JVM bytecode
Aside from Java itself, Scala is by far the most popular of the many JVM languages. If you're developing for Android, or a similar JVM-only platform, or otherwise need out-of-the-box cross-platform compatibility, but the performance of a compiled language, Scala is the way to go.
Pro Very good online courses
On coursera you can find great introduction to Scala by Martin Odersky.
Pro Type inference leads to a simpler syntax
Pro Expressive functional programming abstraction for reusable and safe code
Cons
Con Many errors pass silently
JavaScript looks for every possible way to treat the code you write as runnable and is very reluctant to point out likely errors. For example, you have call a function with too many arguments, the extra arguments are simply discarded.
Con Easy to accidentally use globals
If you forget a var or new, you can clobber the global scope. For tiny scripts (JavaScript's original use case) globals are great, but for larger applications globals are generally regarded as a Bad Thing. This is because changes to one small part of a program can randomly break things anywhere else. These kinds of bugs are notoriously hard to find.
Con Does not teach you about data types
Since JavaScript is a dynamically typed language, you don't have to learn about data types if you start using JavaScript as your first language. Data types being one of the most important concepts in programming. This will also cause trouble in the long run when you will have to (inevitably) learn and work with a statically typed language because you will be forced to learn the type system from scratch.
Con Weird type coercions
'5' - 1 == 4, but '5' + 1 == 51. There are other examples that make even less sense.
Con The "this" keyword doesn't mean what you think it means
this is bound to whatever object called the function. Unless you invoke it as a method. Unless you invoke it as a constructor. Unless it's an arrow function.
Con Complex
JavaScript has a long litany of warts and gotchas. JavaScript will bite you if you're just a wee bit careless. You have to exercise a great deal of caution and awareness. You either have to know the entire 545-page ES6 spec to avoid them all, or use a linter to help restrict you from using the bad parts (and you still have to be familiar with the language), but beginners don't know these things. (Linters are also prone to time-wasting false positives.) This is a significant cognitive burden on even the experienced programmer (as if coding wasn’t hard enough already), but try learning to program in the first place on top of all of this and you'll understand that JavaScript is a terrible choice for the beginner.
Con Each browser has its own quirks when executing the same code in some cases
Beginner programmers often make the mistake of coding something, seeing it works in the browser they tested it in, and then scratching their heads when it doesn't work in another browser. Ideally you'd want a language that works consistently across all platforms in order to be able to focus more on the programming and less on the underlying environment. It just takes time away from learning and forces you to spend time figuring out why this worked in browser X but not browser Y.
Con Asynchronous coding is not easy for beginners
JavaScript can work synchronously but its current use is mainly around asynchronous instructions, and it's surely not a good way to start learning programming.
Con Array-like objects
Many cases when you should get an Array, you just get an Array-like object instead and none of the Array methods work on it.
Con Limited standard library
Much often needed functionality is not in the standard library. (Contrast with Python.) However, there are well established libraries such as Lodash, to fill the gap (however, due to the diverse/fractured ecosystem it may not be clear what library to use).
Con The `null` and `undefined` objects aren't really objects
Therefore, attempts to call methods on them will fail. This is especially frustrating since these are often returned instead of throwing exceptions. So a failure may appear far away from the actual cause, which makes bugs very hard to find.
Con Numbers that begin with 0 are assumed octal
This is very surprising for beginners. Especially since 00 through 07 seem to work just fine. And this isn't just for hardcoded numbers. The parseInt() function has the same problem, but only on some systems.
Con Fast moving
The language and the web platform move fast these days. this makes it difficult for students as there is a lot of fragmentation and outdated information.
Con Many tutorials, code, and resources, are structured for older ES5 code
Con The constant churn of tooling and language
Trying to keep up=javascript fatigue. You won't have time to learn anything else if this is your first language, and you will probably think all programmers are crazy. Plus web assembly may open the door for better alternatives.
Con Easy to fall into bad manners and bad mind structure
It wouldn't consolidate a good mind structure for moving to other languages. Too open.
Con Good tools are pretty much a MUST for new programmers
You really want to be using a good editor (light IDE) and a linter, type checker (e.g.:Flow), etc. until you grok the language. And choosing / setting-up that development environment is it's own learning curve. If taught in a classroom, using a subset of JS with solid tools, there is an argument that JS could be an ideal first language... however, that is a lot of ceremony to protect the new programmer from JS gotchas. But without the tools, JS can be a very painful painful first language (trying to figure out why your code isn't doing what you intended).
Con Has really bad parts you're better off avoiding altogether
But beginners won't know better. And even after you learn, you might have to deal with others' code that uses the bad stuff. JavaScript was originally developed in 10 days. It just wasn't designed that carefully.
Con Counter-intuitive type conversion
JavaScript is rather inconsistent when dealing with different types. For example, when working with the + operator and two numbers, they will be added, two strings will be concatenated:
3+5; // 8;
"Hello "+"world"; // "Hello world"
When using + with a string and a non-string operand, the non-string operand is converted to a string and the result is concatenated:
"the answer is "+42; // "the answer is 42"
"this is "+true; // "this is true"
In any other case (except for Date) the operands are converted to numbers and the results are added:
1+true; // = 1+1 = 2;
null+false; // = 0+0 = 0;
Con Fractured ecosystem
Angular, React, Ember, Meteor, Backbone, Knockout, Express, Mithril, Aurelia. The web frameworks pass in and out of fashion too quickly to keep up with. The endless civil wars are becoming tiresome.
Con Can be intimidating for beginners
Scala is an industrial language. It brings functional programming to the JVM, but not with a "start small and grow the language" perspective, but rather a very powerful language for professional programmers.
Con Static type system inherits cruft from Java
The type system is too complicated yet still less powerful than Haskell's.
Con Way too complex for beginners
Even for seasoned programmers it's a difficult language.