id: asts

sidebar_label: ASTs

Parsers such as those in ESLint and TypeScript read in the text of source code and parse it into a standard format they can reason about known as an **Abstract Syntax Tree** (AST).

ASTs are called such because although they might contain information on the location of constructs within source code, they are an abstract representation that cares more about the semantic structure.

ESLint's built-in parser that outputs an `estree`-shaped AST is also a separate package, called **[`espree`]**.

You can play more with various ASTs such as ESTree on [astexplorer.net] and read more details on their [Wikipedia article](https://en.wikipedia.org/wiki/Abstract_syntax_tree).

[astexplorer.net]: https://astexplorer.net

In some cases, ESLint provides a rule itself, but it doesn't support TypeScript syntax; either it crashes, or it ignores the syntax, or it falsely reports against it.

In these cases, we create what we call an extension rule; a rule within our plugin that has the same functionality, but also supports TypeScript.

This rule allows you to enforce conventions for any identifier, using granular selectors to create a fine-grained style guide.

Instead of an array, you may also pass `null`. This signifies "this selector shall not have its format checked".

This can be useful if you want to enforce no particular format for a specific selector, after applying a group selector.

The `custom` option defines a custom regex that the identifier must (or must not) match. This option allows you to have a bit more finer-grained control over identifiers, letting you ban (or force) certain patterns and substrings.

Accepts an object with the following properties:

- `regex` - a string that is then passed into RegExp to create a new regular expression: `new RegExp(regex)`

- `match` - true if the identifier _must_ match the `regex`, false if the identifier _must not_ match the `regex`.

The `filter` option operates similar to `custom`, accepting the same shaped object, except that it controls if the rest of the configuration should or should not be applied to an identifier.

Example of code for the `{ "onlyInlineLambdas": true }` options:

log();

{