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#1191 in Parser implementations

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Used in 3 crates

MIT license

16K SLoC

Ezno's Parser

Contains "string to AST" parser, AST definitions, AST back to text/string form methods and hooks for traversing/visiting AST. Used in ezno-checker and the Ezno toolchain.

parser lines of code crates.io badge docs.rs badge

use ezno_parser::{ASTNode, Expression};

fn main() {
	let expressions = [
		"4 + 2 * 5",
		"4 * 2 + 5",
		"4 * 2 * 5",
		"console.log(4 * 2, t ? true : `Hi`) == 2 && 4 == 2",
	for expression in expressions {
		let expression = Expression::from_string(expression.to_owned(), Default::default());

Also checkout other parsers such as boa, biome, oxc and swc.


  • Keep under 15k lines of code (excluding /tests and /examples folders)
  • Easy to use API (see /tests and /examples folders for example usage)
  • Run in WASM
  • Keep readable and maintainable
  • Designed for analysis and transformations
    • See expression identifiers can be used to bind information to
    • Retain source positions for use in analysis diagnostics and generating source maps
    • All AST should be visitable. Immutably to collect facts or mutable to transform/remove
  • Optionally via configuration extend the ECMAscript language definition
    • TypeScript type annotations - Interfaces, enums and type alias statements - Parameter, return type and variable annotations - satisfies and as 😑
    • JSX support - Includes HTML comments, special handing of self closing tags from the specification
    • Others under feature = "extras" 👀
  • Transformation and visiting
    • See example
    • ezno-parser-visitable-derive is a macro that automates/generates the visiting implementation
    • The generator macro also makes creating AST easy. See example
  • Positions in source
    • All syntax has reference to where it was in the source using a Span. This uses the source-map crate crate which makes it trivial to build source maps.
  • Partial AST
    • Most of the parser requires valid input. However under a option you can enable a option which can add marked nodes for cases where a expression might be missing. This allows tools that require an AST to work with a source that is still being edited (such as in a LSP).
    • It checks two cases, if either of these cases is encountered, then it adds a marker node: (1) Whether the expression ends in ) etc (useful in if etc) or (2) Whether the next token is const etc and on the next line
  • Output
    • Stripping type annotations can be stripped from output using ToStringOptions { include_types: false, ..Default::default() }
    • Adding indentation under pretty: true, not adding whitespace for production builds
    • Support for source map mapping generation


  • CSTs, close to source operations etc
    • Source with unbalanced parenthesis/brackets
  • Increase code size or decrease readability for minor speed improvements


If in main root rather than this folder, add -p ezno-parser after cargo run to the following commands.

For testing whether the parser can lex a file

cargo run --example lex path/to/file.js

and parse

cargo run --example parse path/to/file.js

Note the Ezno CLI includes the ast-playground subcommand: a more user oriented version of these commands


~94K SLoC