And what good comes out of this mix?

• A parser which already matches the performance of Esprima.NET, while doing more: it also passes the complete Test262 test suite for ECMAScript 2023.
• It is also more economic with regard to stack usage, so it can parse ~1.7x deeper structures.
• More options for fine-tuning parsing.
• A standalone tokenizer which can deal with most of the ambiguities of the JavaScript grammar (thanks to the clever context tracking solution implemented by acornjs).
• As the parser tracks variable scopes to detect variable redeclarations, it will be possible to expose this information to the consumer.

Getting started

1. Install the package from NuGet.

Or, if you want to use additional features like AST to JavaScript or AST to JSON conversion:

2. Import the Acornima namespace in your application

3. Create a parser instance

Or, if you want to tweak the available settings:

4. Use it to parse your JavaScript code:

AST

Legend:

• v - A visitation method is generated in the visitors for the node type.
• s - The node class is sealed. (It's beneficial to check for sealed types when possible.)
• t - The node type (the value of the Node.Type property) as specified by ESTree (shown only if it differs from the name of the node class).
• x - The node class can be subclassed. (The AST provides some limited extensibility for special use cases.)

JSX

The library also supports the syntax extensions JSX. However, mostly for performance reasons, the related functionality is separated from the core parser: it is available in the Acornima.Extras package, in the Acornima.Jsx namespace.

Installation & usage

After installing the Acornima.Extras package as described in the Getting started section, you can parse JSX code like this:

AST

Benchmarks

Known issues and limitations

Regular expressions

The parser can be configured to convert JS regular expression literals to .NET Regex instances (see ParserOptions.RegExpParseMode). However, because of the fundamental differences between the JS and .NET regex engines, in many cases this conversion can't be done perfectly (or, in some cases, at all):

• Case-insensitive matching won't always yield the same results. Implementing a workaround for this issue would be extremely hard, if not impossible.
• The JS regex engine assigns numbers to capturing groups sequentially (regardless of the group being named or not named) but .NET uses a different, weird approach: "Captures that use parentheses are numbered automatically from left to right based on the order of the opening parentheses in the regular expression, starting from 1. However, named capture groups are always ordered last, after non-named capture groups." Without some adjustments, this would totally mess up numbered backreferences and replace pattern references. So, as a workaround, the converter wraps all named capturing groups in a non-named capturing group to force .NET to include all the original capturing groups in the resulting match in the expected order. (Of course, this won't prevent named groups from being listed after the numbered ones.) If needed, the original number of groups can be obtained from the returned RegExpParseResult object's ActualRegexGroupCount property.
• The characters allowed in group names differs in the two regex engines. For example a the group name $group is valid in JS but invalid in .NET. So, as a workaround, the converter encodes the problematic group names to names that are valid in .NET and probably won't collide with other group names present in the pattern. For example, $group is encoded like __utf8_2467726F7570. The original group names can be obtained using the returned RegExpParseResult object's GetRegexGroupName method.
• Self-referencing capturing groups like /((a+)(\1) ?)+/ may not produce the exact same captures. RegexOptions.ECMAScript is supposed to cover this, however even the MSDN example doesn't produce the same matches. (As a side note, RegexOptions.ECMAScript is kinda a false promise, it can't even get some basic cases right by itself.)
• Similarily, repeated nested groups like /((a+)?(b+)?(c))*/ may produce different captures for the groups. (JS has an overwrite behavior while .NET doesn't).
• .NET treats forward references like \1(\w) differently than JS and it's not possible to convert this kind of patterns reliably. (The converter could make some patterns work by rewriting them to something like (?:)(\w) but there are cases where even this wouldn't work.)
• Unicode mode issues:
  • There could be false positive empty string matches in the middle of surrogate pairs. Patterns as simple as /a?/u will cause this issue when the input string contains astral Unicode chars. There is no out-of-the-box workaround for this issue but it can be mitigated somewhat using a bit of "post-processing", i.e., by filtering out the false positive matches after evaluation like it's done here. Probably there is no way to improve this situation until .NET adds the option to treat the input string as Unicode code points.
  • Support for Unicode property escapes is pretty limited (see explanation). Currently, only General Category expressions are converted. But even this is not perfect as the result will depend the Unicode version included in the specific .NET runtime which is executing the parser's code.

To sum it up, legacy pattern conversion is pretty solid apart from the minor issues listed above. However, support for unicode mode (flag u) patterns is partial and quirky, while conversion of the upcoming unicode sets mode (flag v) will be next to impossible - until the .NET regex engine gets some improved Unicode support.

Any feedback appreciated, contributions are welcome!