esprima VS espree

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.

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.

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?

How does ESLint work? ESLint uses Espree for Javascript parsing. It uses an AST to evaluate patterns in code. It does all this before runtime.. meaning, without running your Javascript code it will find the bugs, syntax and stylistic errors.

ESLint doesn't process the code into the compiler phases, rather it provides an option to let you specify a Parser. By default, ESLint uses Espree which essentially converts JS source code to AST data structure, so in case you want to write a rule targeting TypeScript source code, you'll need to specify a different parser in your .eslintrc.json configuration file, same applies for different file extension, for HTML you might use this or creating your own parser!