esprima VS astexplorer

Focusing again on ESLint, the parser used by the linter is called Espree. This is an in-house parser built by the ESLint folks to fully support ECMAScript 6 and JSX on top of the already existing Esprima. The Espree module provide APIs for both tokenization and parsing that you can easily test out.

For TypeScript we have used AST transforms from their compiler API, and for plain JavaScript we did a similar thing using ESPrima. This helped us implement some simple optimizations like stream fusion (combining .filter and .map into a single operation) or avoiding extra object allocations in vector math, which led to nice performance improvements in code that does heavy computation (we process large amounts of data on the server and store results of physics simulations).

I used ESPrima, but any parser would do in this case. I then wrote a simple function to extract all "atomic" non-boolean expressions from it.

AST is an acronym for Abstract Syntax Tree. It's the way to represent code in a format that tools can understand. Usually in form of tree data structure. We will use AST in the format of an Esprima, which is a JavaScript parser that outputs AST.

esprima

Create an AST: Esprima

The thread was originally about CRA vs Vite size on disk (or implicitly, if we're applying it to real world applications, network cost in CI job startup times). And like I said, surrogate pairs don't apply to ASCII.

See this[0] for reference. Note how the first byte must fall within a certain range in order to signal being a surrogate pair. This fact is taken advantage of by JS parsers to make parsing of ASCII code faster by special casing that range, since checking for a valid character in the entire unicode range is quite a bit more expensive[1].

[0] https://github.com/jquery/esprima/blob/0911ad869928fd218371b...

[1] https://github.com/jquery/esprima/blob/0911ad869928fd218371b...

If you want to learn more about parsing, reading the code of an actual recursive parser might be a better idea. Esprima is a decent place to start if you're interested in JS grammar. Then you can look at the babel handbook to learn more about AST transformations. From there, the literature gets quite a bit more heavy. If you get this far and are willing to push further, you'll probably want to grab yourself a copy of the dragon book at a minimum.

You can play with your code here, and visualise ASTs for the same.

Website

To understand this syntax, I recommend exploring AST Explorer. You will have a better view of how the AST of JavaScript works and how to correlate it with the Eslint syntaxy:

You can never mistake type_declaration with an identifier, otherwise the program will not work. Aside from that constraint, you are free to name them whatever you like, there is no one standard, and each parser has it own naming conventions, unless you are planning to use something like LLVM. If you are interested, you can see examples of naming in different language parsers in the AST Explorer.

The rule that I want to write will be called not-allows-underscore: the idea is to abolish the use of underscores when declaring variables or functions. It's a real dummy rule, but it should be enough to see in action the concepts that we have discussed earlier. The first thing that I would do is to go to AST Explorer, write down a code that declares variables and functions (both standard and arrows one) and take a look at what type of node is the one that encodes the identifier. Doing that, I found out that the node type of my interest is Identifier, what a surprise! 🤣. In particular, the structure of the node holds the string used as identifier in the name property.

All the information about the API, AST node names, AST Explorer, etc. you can read in the official documentation. I’m just going to show examples of how to automate the check-up of our created conventions.

AST Viewer

If you want to see how the AST of popular languages looks, I recommend the AST Explorer. It supports various languages, and you can view the complete AST and navigate through the nodes. If you want to go further, you can try to copy some logic from an existing parser and implement it in your own, such as calculating an expression according to precedence order, for example: 1 + 2 * 3 (which is 7, not 9).

https://astexplorer.net/ is a good resource/playground for understanding ASTs and transpilers.