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What exactly does "Javascript can be run outside the browser" mean in plain English? [duplicate]

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What is client side javascript and what is server side javascript?
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I'm learning Javascript, and I keep seeing this phrase being used when speakers are explaining NodeJS and the V8 engine in relation to Javascript. I know because of Node, we can run JS "outside of the browser," but what does that mean? Oftentimes, instructors use terms I don't know yet (like "server-side scripting") to explain, which makes it difficult to understand.
From what I know so far, it means that you can use JS like a back-end language..? Whereas "running inside a browser," means you can only code visual/front-end interactions, things only users see. Am I correct?

Yes you are more or less on the right track. The V8 engine is what powers both chrome and nodejs. When used with nodejs, it provides a runtime environment for javascript that allows JS code to be run "outside the browser". Traditionally the only place javascript could run was in a browser that implemented support for ECMAScript (all major browsers). But then along came node.js, which pulled out the critical v8 engine from chrome, and mixed with LibUV for C++ support, created the first runtime that successfully allowed Javascript to run without a browser to run the code. With Node.js (and Deno if you are curious) we can implement a variety of "back-end", server-side, code. Think anything you could program in another language now available to be programmed in Javascript. Node.js and Express js are absolutely great for running web servers, microservices, and many other services independent from a browser run time environment.

Originally, the ability to process the JavaScript language was something that Netscape added to their browser so that it could process not only HTML and CSS, but also JavaScript. Microsoft followed suit and so did all the other browsers. That's what we call running JavaScript within the browser.
But (as an example) Node.js is a command line runtime that includes the core JavaScript runtime, but extracted from the Chrome browser itself. So, with Node, you have the ability to write and run JavaScript, but you aren't running it inside of a browser and therefore you don't have the Document Object Model (DOM) or the Browser Object Model (BOM) available to you because those are APIs that are native to browsers.
But you do have the core JavaScript language and can create applications that do all sorts of non-browser tasks.
You'll find that many other development environments and tools have similar set ups, where the core JavaScript runtime has been added so that you can develop with JavaScript, but outside of a browser.

Difference between JavaScript shell and node.js

I have been developing in Node.js for some time now.
Today, I came across this article
Introduction to the JavaScript shell - Mozilla | MDN
It talks about javascript shell and goes onto say that it can execute javascript programs from a file as well.
I was able to research and understand V8 and spydermonkey.
I want to know the difference between Node.js and the javascript shell talked about in this article since it says that the shell can execute javascript programs on its own.
Do they only differ in that the node.js uses a V8 engine while the other uses a spidermonkey?
if so then why is it that node.js is so popularly used for writing Server Side JavaScript?
I couldn't exactly find what I was looking for on the Internet. either google showed me difference between spidermonkey and v8 or some forums on "difference between javascript and node.js" and since I am a new developer its really hard for me to understand,
Can spidermonkey be used to achieve the same?

JavaScript is a language.
node.js is not a language or a special dialect of JavaScript - it's just a thingamabob that runs normal JavaScript.
All browsers have JavaScript engines that run the JavaScript of web pages. Firefox has an engine called Spidermonkey, Safari has JavaScriptCore, and Chrome has an engine called V8.
Node.js is simply the V8 engine bundled with some libraries to do I/O and networking, so that you can use JavaScript outside of the browser, to create shell scripts, backend services or run on hardware (https://tessel.io/).
Credits : https://www.quora.com/What-is-the-difference-between-JavaScript-and-Node-js
I hope that helped clearing out the basic difference between them. The specifics you required are not answered here.

Node.js enables JavaScript to be used for server-side scripting, and
runs scripts server-side to produce dynamic web page content before
the page is sent to the user's web browser.
Source: https://en.wikipedia.org/wiki/Node.js
Obviously the shell can not serve HTML web pages by itself.
In addition, Node.js is asynchronous, and non-blocking, meaning it can serve multiple requests and host multiple processes simultaneously.
EDIT: provided source.

How is Javascript translated to bytecode?

I can't find information on the web how W3C languages compile to machine code. I know that the gap between the web and the processor must be somehow the browser, but how does it work and what are the steps till Javascript is executed in the processor?
Links to scientific documents would be also greatly appreciated.

It's up to the implementation; the specification is the full description of the language and how it's supposed to work, implementations are free to satisfy that implementation in any way they like. Some implementations seem (from the outside) to run it purely as an interpreter in the old sense; others may or may not compile to bytecode; V8 (the JavaScript engine in Chrome, Chromium, Brave, Node.js, and others) used to compile to machine code (twice, for hotspots in the app), but now starts out parsing to bytecode and running it in an interpreter and only compiling hotspots as necessary (details). (There's also a V8 mode where it only interprets, which they're experimenting with for environments where compiling at runtime isn't an option, such as iOS where non-Apple apps aren't allowed to allocate executable memory.)
The V8 team (V8 being the JavaScript engine in Chromium and Chrome) periodically publish descriptions of how they get the fantastic speed out of V8 that they do. You may find some of that on the V8 blog.
Naturally, you can also kick around the code of any of the open-source implementations. V8 and SpiderMonkey (Mozilla's engine) are the two major open-source ones I know.

This may help : http://www.ecma-international.org/publications/standards/Ecma-262.htm
There is no spec for how to translate into bytecode (That is up to the browser developers) but there are specs about how the language should behave

For Firefox there's some specifications on its bytecodes:
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Bytecodes
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/Bytecode
For V8 it's compiled to native code directly:
http://jayconrod.com/posts/51/a-tour-of-v8-full-compiler

Javascript (as it's name suggests) is a dynamic scripting language. Meaning that it's code is analysed and executed at runtime by the web-browser's Javascript engine.
It is up to the Web-browser, how it wants to deal with Javascript. Some may generate an intermediate language, or bytecode. Some may directly analyse and execute it.
Here are the steps to the simplest way to execute Javascript (parsing and executing at runtime):
Parsing and Preprocessing (recursive descent or otherwise)
Analysis
Execution
Chrome's Javascript Engine compiles Javascript to native platform-specific machine code (for optimum performance) It also has a Garbage Collection Mechanism.

In addition to the useful, specific answers already given, the phrase 'adaptive optimisation' is probably worth looking up if performance is your main interest. JavaScript and its interpreters are just the latest instance of systems that need to convert something else to machine code at runtime, so there's plenty of wider reading. The bytecode forms of Pascal, Smalltalk, Java, etc can easily enough be viewed as an intermediate form in the process of running a defined language on arbitrary hardware — Apple's SquirrelFish explicitly creates a bytecode and uses a JIT compiler on that, for example.

Javascript Engines Advantages

I am confused about JavaScript engines right now. I know that V8 was a big deal because it compiled JavaScript to native code.
Then I started reading about Mozilla SpiderMonkey, which from what I understand is written in C and can compile JavaScript. So how is this different from V8 and if this is true, why does Firefox not do this?
Finally, does Rhino literally compile the JavaScript to Java byte code so you would get all the speed advantages of Java? If not, why do people not run V8 when writing scripts on their desktops?

There are various approaches to JavaScript execution, even when doing JIT. V8 and Nitro (formerly known as SquirrelFish Extreme) choose to do a whole-method JIT, meaning that they compile all JavaScript code down to native instructions when they encounter script, and then simply execute that as if it was compiled C code. SpiderMonkey uses a "tracing" JIT instead, which first compiles the script to bytecode and interprets it, but monitors the execution, looking for "hot spots" such as loops. When it detects one, it then compiles just that hot path to machine code and executes that in the future.
Both approaches have upsides and downsides. Whole-method JIT ensures that all JavaScript that is executed will be compiled and run as machine code and not interpreted, which in general should be faster. However, depending on the implementation it may mean that the engine spends time compiling code that will never be executed, or may only be executed once, and is not performance critical. In addition, this compiled code must be stored in memory, so this can lead to higher memory usage.
The tracing JIT as implemented in SpiderMonkey can produce extremely specialized code compared to a whole-method JIT, since it has already executed the code and can speculate on the types of variables (such as treating the index variable in a for loop as a native integer), where a whole-method JIT would have to treat the variable as an object because JavaScript is untyped and the type could change (SpiderMonkey will simply "fall off" trace if the assumption fails, and return to interpreting bytecode). However, SpiderMonkey's tracing JIT currently does not work efficiently on code with many branches, as the traces are optimized for single paths of execution. In addition, there's some overhead involved in monitoring execution before deciding to compile a trace, and then switching execution to that trace. Also, if the tracer makes an assumption that is later violated (such as a variable changing type), the cost of falling off trace and switching back to interpreting is likely to be higher than with a whole-method JIT.

V8 is the fastest, because it compiles all JS to machine code.
SpiderMonkey (what FF uses) is fast too, but compiles to an intermediate byte-code, not machine code. That's the major difference with V8. EDIT- Newer Firefox releases come with a newer variant of SpideMonkey; TraceMonkey. TraceMonkey does JIT compilation of critical parts, and maybe other smart optimizations.
Rhino compiles Javascript into Java classes, thus allowing you to basically write "Java" applications in Javascript. Rhino is also used as a way to interpret JS in the backend and manipulate it, and have complete code understanding, such as reflection. This is used for example by the YUI Compressor.
The reason why Rhino is used instead of V8 all over the place is probably because V8 is relatively new, so a lot of projects have already been using Rhino/Spidermonkey as their JS engine, for example Yahoo widgets. (I assume that's what you're referring to with "scripts on their desktops")
edit-
This link might also give some insight of why SpiderMonkey is so widely adopted.
Which Javascript engine would you embed in your application?

If you want to see how the various in-browser Javascript engines stack up, install Safari 4 (yes it runs on Windows now too!), Chrome V8, Firefox 3.5, and IE 8 (if you are on windows) and run the benchmark:
http://www2.webkit.org/perf/sunspider-0.9/sunspider.html
I believe as Pointy said above, the new Firefox 3.5 uses TraceMonkey that also compiles to intermedit code on the fly using some form of JIT. So it should compare to V8 somewhat favorably. At least it won't be 10x slower than V8 like Firefox 3 SpiderMonkey (without JIT) was.
For me... safari 4.0.3 was 2.5x faster than Tracemonky in Firefox 3.5.3 on Win XP. IE8 was much much slower. I don't have Chrome installed at the moment.
Don't know about Rhino compiling to java bytecode. If it's still interpreting the dynamic features of Javascript such as being able to add attributes to object instances at runtime (example obj.someNewAttribute="someValue" which is allowed in Javascript)... I'm not so sure that it's entirely "compiled" to bytecode, and you might not get any better performance other than you don't have to compile from Javascript source code text every time your Javascript runs. Remember that Javascript allows very dynamic syntax such as eval("x=10;y=20;z=x*y"); which means you can build up strings of code that get compiled at runtime. That's why I'd think Rhino would be mixed-mode interpreted/compiled even if you did compile to JVM bytecode.
The JVM is still an interpreter, albeit a very good one with JIT support. So I like to think of Rhino-on-JVM as 2 interpreter layers (interpreter-on-interpreter) or interpreter^2. Whereas most of your other Javascript engines are written in C, and as such should perform more like interpreter^1. Each interpreter layer can add 5-10x performance degradation as compared to C or C++ (ref Perl or Python or Ruby for example), but with JIT the performance hit can be much lower on the order of 2-4x. And the JVM has one of the most robust & mature JIT engines ever.
So your mileage will definitely vary and you probably would benefit from doing some serious benchmarks if you want a real answer for your intended application on your own hardware & OS.
Rhino can't be too awfully slow, since I know lots of folks are using it. I think it's main appeal is not its speed, but the fact that is easy-to-code/light-weight/embeddable/interpreter that has hooks into Java libraries, which makes it perfect for scripting/configuration/extensibility of your software project. Some text editors like UltraEdit are even embedding Javascript as an alternative macro scripting engine. Every programmer seems to be able to stumble through javascript pretty easily, so it's easy to pick up as well.
One advantage to Rhino is that it runs pretty much anywhere the JVM runs. In my experience, trying to get stand-alone TraceMonkey or SpiderMonkey to build & run from command line can be a bit painful on systems like Windows. And embedding in your own application can be even more time consuming. But the payback to having an embeddable language would be worth it for a big project, as compared to having to "roll your own" mini scripting solution if that's what you're looking to do.
Scripting with Rhino is really easy if you have Java and the rhino jar, you just write your javascript and run it from command line. I use it all the time for simple tasks.

To respond the question, why native code Vs Byte code...
The native code is faster and for google a strategic choice because they have plan to JS one of them at least is ChromeOS.
A good video about this question is posted on Channel9 with an interview with Lars Bak the man behind V8 can be found here

2022 Update
V8 as also SpiderMonkey do support MultiPass Compilation as also SinglePass Compilation.
Java Rhino got deprecated and replaced by a new concept called Truffle Framework it is part of the GraalVM Stack which is in fact a JVMCI added to the JVM where CI means Compiler Interface. That allows the Truffle Language Implementer Framework to Implement any Language and translate that to Java Byte Code in MultiPass and Single Pass. the ECMAScript implementation is called GraalJS and it comes in 2 Flavors
GraalJS the 1:1 Rhino Replacement including compatability mode for incremental adoption and graal-node a NodeJS Fork where v8 is replaced with the GraalJS Engine.
as sayed Truffle is a Polyglot Language Implementer Framework so that works with any GraalVM Supported Language.
hope that helps and makes some sense since the jar versions do most best work when the whole stack has aligned versions you should alaways install the complet stack:
https://github.com/graalvm/graalvm-ce-builds/releases
download and install that then use the so called gu command to add graaljs
follow the instructions here: https://github.com/oracle/graaljs
Yes GraalVM can in general Compile Single Binary Executeable Files of all Polyglot Languages this is called AOT Compilation it has faster boot times but less throughput it can also do JIT and it can do a mix of both.

Building v8 without JIT

I would like to run some tests on v8 with and without JIT to compare
performances.
I know JIT will improve my average speed performance, but it would be
nice for me to have some actual more detailed tests results as I want to work with mobile platforms.
I haven't found how to enable or disable JIT like it exists on Squirrelfish (cf. ENABLE_JIT in JavaScriptCore/wtf/Platform.h).
Does somebody knows how to do that with v8?
Thanks.
Alexandre

For those who may be interested I got the following answer from Søren Gjesse on v8-users google groups.
Hi,
It is not possible to disable dynamically generated code in V8. Unlike other
JavaScript engines V8 does not have an interpreter mode - it always
generates native code.
One exception is the RegExp engine which have an interpreter and native code
version. The compile time define V8_NATIVE_REGEXP enables generating native
code for RegExp processing.
Regards,
Søren

On a 32 bit little endian platform you can build with the ARM simulator. This is a testing feature and performance will be terrible, but it does work and involves no direct execution of JITed code. Use the --simulator=arm option on the scons or test.py command line.