escodegen VS espree

What's cool about Esprima syntax is that there are tools that generate code based on their AST. An example is escodegen which takes Esprima AST as input and outputs JavaScript code. You can think that you can use just strings to generate code, but this solution will not scale. In this tutorial, I show only a single if statement but you will run into a lot of problems if you will have more complex code.

Unparse the modified AST back into Javascript: Escodegen

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!