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LibWeb Code Style & Patterns

This document aims to describe agreed upon code style and patterns used across LibWeb.

Directory Structure

Generally we use a subdirectory, and thus C++ namespace, for each individual spec. For example XHR (xhr.spec.whatwg.org):

  • Code lives in LibWeb/XHR/
  • Uses the C++ namespace Web::XHR

If necessary, code can also be grouped into sub-subdirectories (for example HTML/Scripting/).

Sometimes a spec document affects several areas of the web platform. An example of this is CSSOM, which of course contains features belonging in LibWeb/CSS/ / Web::CSS, but at the same time implements additions to Window from the HTML spec. Use best judgement in those cases.

Error Handling

The following error types are commonly used in LibWeb, each having a different purpose:

AK::ErrorOr<T>

This error type is generally only used to propagate OOM errors from AK and other general libraries. It should not be used to propagate any other kinds of errors, even though this is supported and used in other parts of the system. For LibWeb, use any of the error types below.

This should be propagated as far as possible before being turned into a JS error object (via the TRY_OR_THROW_OOM macro). This avoids a situation where almost everything returns a generic WebIDL::ExceptionOr<T>.

Web::WebIDL::ExceptionOr<T>

This is the most common and at the same time most broad error type in LibWeb. Internally it stores a variant of supported errors:

  • SimpleException
  • JS::NonnullGCPtr<DOMException>
  • JS::Completion (from JS::ThrowCompletionOr<T>, assumed to be of Type::Throw)

Use this error type for anything that needs to interact with the JS bindings, which will generally know how to turn any of the internally supported errors into JS objects.

Web::WebIDL::SimpleException

This is a thin wrapper around various built-in errors from ECMAScript:

  • EvalError
  • RangeError
  • ReferenceError
  • TypeError
  • URIError

Instead of constructing one of these directly, create a SimpleException with the appropriate type and message instead whenever required by a web spec. These will be converted into actual JS objects in the bindings layer.

Note Relevant WebIDL documentation: https://webidl.spec.whatwg.org/#dfn-simple-exception

Web::WebIDL::DOMException

This is an error type from the WebIDL spec specifically for web AOs where none of the JS built-in error types are sufficient. Like SimpleException, use when indicated by a web spec.

Note Relevant WebIDL documentation: https://webidl.spec.whatwg.org/#idl-DOMException

JS::ThrowCompletionOr<T>

This is an error type from LibJS, which uses "completions" to propagate errors and other types of AO results. Don't use this in LibWeb unless absolutely necessary, e.g. when overriding a JS::Object virtual method that returns this type.

At the call site, these should be wrapped in a ExceptionOr<T> as soon as possible for further propagation.

Note Relevant ECMAScript documentation: https://tc39.es/ecma262/#sec-completion-record-specification-type

Comments

As in LibJS, all functions that represent an operation or JS function from a web specification must have:

  • A spec link, above the function definition:

    // https://fetch.spec.whatwg.org/#concept-fetch
WebIDL::ExceptionOr<JS::NonnullGCPtr<Infrastructure::FetchController>> fetch(JS::Realm& realm, Infrastructure::Request& request, Infrastructure::FetchAlgorithms const& algorithms, UseParallelQueue use_parallel_queue)
{
    // ...
}

  • Comments for each individual step of the operation:

    // 1. Assert: request’s mode is "navigate" or processEarlyHintsResponse is null.
VERIFY(request.mode() == Infrastructure::Request::Mode::Navigate || !algorithms.process_early_hints_response().has_value());

// 2. Let taskDestination be null.
JS::GCPtr<JS::Object> task_destination;

// ...
    • If a step cannot be implemented at the time, prepend it with a FIXME
    • Add a blank line between code and the next comment

  • Optimizations (e.g. fast paths) should be marked as such with an // OPTIMIZATION: comment explaining the reasoning

  • When adding non-standard code for a feature that is otherwise well-specified, it should be marked as such. This does not universally apply as certain areas (layout and painting, for instance) are only broadly spec'd

  • If the spec has additional prose before or after its algorithm steps, that doesn't need to be copied into the code

JS Interfaces

IDL

Try to copy IDL verbatim, only making changes where necessary (IDL parser shortcomings, non-standard extended attributes).

This includes not reordering functions, changing parameter names, etc.

The major difference to the spec is that we use four spaces for indentation, like for any other code (not two).

C++ Naming

Try to stick with the interface's exact name as much as possible for the class and file names (no Object suffixes like in LibJS, for example). When there's a name clash, try to introduce a nested namespace, e.g. Fetch::Request vs Fetch::Infrastructure::Request.

File placement

The .cpp, .h, and .idl files for a given interface should all be in the same directory, unless the implementation is hand-written when it cannot be generated from IDL. In those cases, no IDL file is present and code should be placed in Bindings/.