astexplorer VS Acorn

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.

Why do we then have other JavaScript parsers like babel parser, swc parser, acorn, espree and the likes since JavaScript engines have their own internal parsers?

If you're trying to determine if a string is valid Javascript then your best bet is to use a JS parser like Acorn. Trying to DIY parsing with just regex is a good way to end up incredibly frustrated.

To render UI I use Svelte. To parse JS files I use Acorn JS in Web Worker.

Behind Babel is created by an Australian developer Sebastian McKenzie and now consists a small team of contributors. Babel also makes uses a polyfill library called core-js, which in turn uses a JavaScript parser, acorn.

At line 3, we import parseScript from meriyah which is a JavaScript parser (could have been babel, acorn, swc, etc doesn't matter for our use case as long as they correctly implement the spec).

A simple starting point is Acorn with estree-walker. Not sure if you'll need more advanced functionality but that pair has worked well for my usage. It's also the combo that Svelte uses internally.

https://github.com/acornjs/acorn For parsing the tree