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LibJS: Annotate Promise implementation with spec comments

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I wanted to do this for a long time. The guts of Promise are pretty
complex, and it's easier to understand with the spec right next to it.
Also found a couple of issues along the way :^)
This commit is contained in:
Linus Groh 2021-11-14 00:33:16 +00:00
parent df06552b48
commit 01c2570678
7 changed files with 523 additions and 41 deletions
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32
Userland/Libraries/LibJS/Runtime/PromiseReaction.cpp
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@ -16,41 +16,63 @@ namespace JS {
ThrowCompletionOr<PromiseCapability> new_promise_capability(GlobalObject& global_object, Value constructor)
{
auto& vm = global_object.vm();
// 1. If IsConstructor(C) is false, throw a TypeError exception.
if (!constructor.is_constructor())
return vm.throw_completion<TypeError>(global_object, ErrorType::NotAConstructor, constructor.to_string_without_side_effects());
// 2. NOTE: C is assumed to be a constructor function that supports the parameter conventions of the Promise constructor (see 27.2.3.1).
// 3. Let promiseCapability be the PromiseCapability Record { [[Promise]]: undefined, [[Resolve]]: undefined, [[Reject]]: undefined }.
// FIXME: This should not be stack-allocated, the executor function below can be captured and outlive it!
// See https://discord.com/channels/830522505605283862/886211697843531866/900081190621569154 for some discussion.
struct {
Value resolve { js_undefined() };
Value reject { js_undefined() };
} promise_capability_functions;
// 27.2.1.5.1 GetCapabilitiesExecutor Functions, https://tc39.es/ecma262/#sec-getcapabilitiesexecutor-functions
// 4. Let executorClosure be a new Abstract Closure with parameters (resolve, reject) that captures promiseCapability and performs the following steps when called:
// 5. Let executor be ! CreateBuiltinFunction(executorClosure, 2, "", « »).
auto* executor = NativeFunction::create(global_object, "", [&promise_capability_functions](auto& vm, auto& global_object) -> ThrowCompletionOr<Value> {
auto resolve = vm.argument(0);
auto reject = vm.argument(1);
// No idea what other engines say here.
if (!promise_capability_functions.resolve.is_undefined()) {
// a. If promiseCapability.[[Resolve]] is not undefined, throw a TypeError exception.
if (!promise_capability_functions.resolve.is_undefined())
return vm.template throw_completion<TypeError>(global_object, ErrorType::GetCapabilitiesExecutorCalledMultipleTimes);
}
if (!promise_capability_functions.reject.is_undefined()) {
// b. If promiseCapability.[[Reject]] is not undefined, throw a TypeError exception.
if (!promise_capability_functions.reject.is_undefined())
return vm.template throw_completion<TypeError>(global_object, ErrorType::GetCapabilitiesExecutorCalledMultipleTimes);
}
// c. Set promiseCapability.[[Resolve]] to resolve.
promise_capability_functions.resolve = resolve;
// d. Set promiseCapability.[[Reject]] to reject.
promise_capability_functions.reject = reject;
// e. Return undefined.
return js_undefined();
});
executor->define_direct_property(vm.names.length, Value(2), Attribute::Configurable);
executor->define_direct_property(vm.names.name, js_string(vm, String::empty()), Attribute::Configurable);
// 6. Let promise be ? Construct(C, « executor »).
MarkedValueList arguments(vm.heap());
arguments.append(executor);
auto* promise = TRY(construct(global_object, constructor.as_function(), move(arguments)));
// 7. If IsCallable(promiseCapability.[[Resolve]]) is false, throw a TypeError exception.
if (!promise_capability_functions.resolve.is_function())
return vm.throw_completion<TypeError>(global_object, ErrorType::NotAFunction, promise_capability_functions.resolve.to_string_without_side_effects());
// 8. If IsCallable(promiseCapability.[[Reject]]) is false, throw a TypeError exception.
if (!promise_capability_functions.reject.is_function())
return vm.throw_completion<TypeError>(global_object, ErrorType::NotAFunction, promise_capability_functions.reject.to_string_without_side_effects());
// 9. Set promiseCapability.[[Promise]] to promise.
// 10. Return promiseCapability.
return PromiseCapability {
promise,
&promise_capability_functions.resolve.as_function(),