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Acorn for .NET

A C# port of the Acorn JavaScript parser, targeting .NET 10. It parses JavaScript/ECMAScript source into ESTree-compatible abstract syntax trees — the same trees the original JavaScript library produces.

This is a fork/port. The original JavaScript sources are kept in the repository root (acorn/, acorn-loose/, acorn-walk/) so the port can be kept in sync with upstream; all C# code lives under dotnet/.

Packages

Package Description
Acorn The main parser and tokenizer.
Acorn.Loose An error-tolerant parser that repairs broken input.
Acorn.Walk A syntax-tree walker for ESTree trees.

Usage

using Acorn;

// Parse a whole program
var options = new Options { EcmaVersion = 2020, Locations = true };
Node ast = Parser.ParseStatic("let answer = 6 * 7;", options);

Console.WriteLine(ast.Type);                 // "Program"
var body = (List<object?>)ast["body"]!;
var decl = (Node)body[0]!;
Console.WriteLine(decl.Type);                // "VariableDeclaration"

// Parse a single expression at an offset
Node expr = Parser.ParseExpressionAt("1 + 2", 0, new Options { EcmaVersion = 2020 });

// Tokenize
var tokenizer = Parser.Tokenizer("a + b", new Options { EcmaVersion = 2020 });
for (Token t = tokenizer.GetToken(); t.Type != TokenTypes.Eof; t = tokenizer.GetToken())
    Console.WriteLine($"{t.Type.Label} {t.Start}..{t.End}");

Options

Acorn.Options mirrors the JavaScript options: EcmaVersion (an int such as 5, 2015, 2020, or Options.Latest), SourceType ("script", "module", "commonjs"), Locations, Ranges, PreserveParens, AllowReturnOutsideFunction, AllowImportExportEverywhere, AllowAwaitOutsideFunction, AllowReserved, OnComment, OnToken, and more.

The AST (Node)

Because acorn assigns node properties dynamically, Node is a property bag: Type, Start, End, Loc, Range are strongly-typed members; all other properties are reached through the string indexer, e.g. node["name"], node["body"], node["value"]. Child arrays are List<object?>; nested locations are SourceLocation/Position.

Loose parsing

using Acorn.Loose;

Node ast = AcornLoose.Parse("foo(", new Options { EcmaVersion = 2020 });
// Produces a best-effort tree instead of throwing.

Walking

using Acorn.Walk;

AstWalker.Simple(ast, new Dictionary<string, Action<Node, object?>>
{
    ["CallExpression"] = (node, state) => Console.WriteLine("call at " + node.Start)
});

Building & testing

cd dotnet
dotnet build Acorn.sln -c Release
dotnet test test/Acorn.Tests/Acorn.Tests.csproj

The test suite runs the C# parser against 3530 conformance cases generated from acorn's own JavaScript test suite (see ../CLAUDE.md for how). To regenerate the fixtures after an upstream sync:

node tools/gen-fixtures.js

License

MIT, the same as upstream Acorn. See ../acorn/LICENSE.