I'm diving into web development and, as I inspect the source of some web pages, I see people referencing "/scripts/compiled.js", which tells me people are compiling their javascript code.
Thanks so much in advance for your help!
Compilation in terms of JavaScript means something slightly different: It usually refers to a file where all the JS used on a site is collected into one file and minimized, not compilation as we understand from compiled languages (that compile to machine language or bytecode).
Compilation of JavaScript code (usually cross-compilation from JavaScript to JavaScript) allows you to detect certain classes of error statically (i.e. without having to run the code) whereas JavaScript for the most part defers all such checks to runtime: this means that rarely executed parts of your script that use a variable incorrectly will only be detected if the code is ever executed. A compiler can detect many usages at compile time before the code is even deployed.
This kind of type checking is achieved by performing type inference: i.e. the compiler propagates known type information about a variable based on the point at which a variable's value is assigned throughout the program's various code paths and determines if the variable is used in a consistent fashion in other areas of the program. Obviously, due to the highly dynamic nature of JavaScript, this type inference is not perfect but it can still be useful.
JavaScript compilers can also perform minimization in order to reduce the size of a script to a minimum by renaming variables and method names etc. in order to minimize bandwidth usage as well as merge multiple scripts into a single file for ease of downloading.
Philosophical Edit: Compilers or cross-compilers that generate JavaScript as their target are effectively treating JavaScript as the "machine language" of the web browser and this is very similar in essence to compilers that emit C as a portable intermediate language. The line between preprocessor and compiler can be a grey one but I would suggest that something that tokenizes, parses and semantically analyses source code and emits some intermediate or machine language at the backend is more correctly considered a compiler than a preprocessor which usually performs only simple text substitution or macro expansion.
"Compiled" code is often minified and compressed. Compressing removes whitespace and minification will attempt to remap functions and variables to smaller names. It is not a traditional compiler which will turn it into machine or bytecode.
function Foo(count, width)
{
setCount(count);
setWidth(width / 2);
}
will turn into
function c(aa,ba){a(aa);b(ba/2);}
Readability is far reduced but can have a significant savings on file size. There are a couple good online minification tools that you can try out.
JavaScript can't be compiled. What they're doing is one of two things: obfuscating and minimizing.
Obfuscating is when variables and function names in code are converted from something meaningful like bindClickListener to something like a. This makes code difficult for humans to read and copy.
Minimizing is simply reducing the amount of whitespace to get the file as compact as possible, such that less bandwidth is consumed for each request (and thus the page loads faster).
"Compiling" is a misnomer. On the web, it almost always refers to "compression", which basically removes unnecessary whitespace and squeezes variable names. (of course, many tools do more than this)
The main benefit of doing this is that it reduces file size, and hence download times, at the cost of losing human-readability.
Two popular tools off the top of my head: