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LibJS: Implement non-value-producing statements properly

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For various statements the spec states:

    Return NormalCompletion(empty).

In those cases we have been returning undefined so far, which is
incorrect.

In other cases it states:

    Return Completion(UpdateEmpty(stmtCompletion, undefined)).

Which essentially means a statement is evaluated and its completion
value returned if non-empty, and undefined otherwise.

While not actually noticeable in normal scripts as the VM's "last value"
can't be accessed from JS code directly (with the exception of eval(),
see below), it provided an inconsistent experience in the REPL:

    > if (true) 42;
    42
    > if (true) { 42; }
    undefined

This also fixes the case where eval() would return undefined if the last
executed statement is not a value-producing one:

    eval("1;;;;;")
    eval("1;{}")
    eval("1;var a;")

As a consequence of the changes outlined above, these now all correctly
return 1.

See https://tc39.es/ecma262/#sec-block-runtime-semantics-evaluation,
"NOTE 2".

Fixes #3609.
This commit is contained in:
Linus Groh 2021-03-16 09:12:34 +01:00 • committed by Andreas Kling
parent dadf2e8251
commit c499239137
4 changed files with 40 additions and 26 deletions
Download patch file Download diff file Expand all files Collapse all files

20
Userland/Libraries/LibJS/Interpreter.cpp
View file

@ -61,6 +61,8 @@ void Interpreter::run(GlobalObject& global_object, const Program& program)
VM::InterpreterExecutionScope scope(*this);
vm.set_last_value({}, {});
CallFrame global_call_frame;
global_call_frame.current_node = &program;
global_call_frame.this_value = &global_object;
@ -73,6 +75,9 @@ void Interpreter::run(GlobalObject& global_object, const Program& program)
VERIFY(!vm.exception());
program.execute(*this, global_object);
vm.pop_call_frame();
if (vm.last_value().is_empty())
vm.set_last_value({}, js_undefined());
}
GlobalObject& Interpreter::global_object()
@ -162,11 +167,10 @@ Value Interpreter::execute_statement(GlobalObject& global_object, const Statemen
auto& block = static_cast<const ScopeNode&>(statement);
enter_scope(block, scope_type, global_object);
if (block.children().is_empty())
vm().set_last_value({}, js_undefined());
for (auto& node : block.children()) {
vm().set_last_value({}, node.execute(*this, global_object));
auto value = node.execute(*this, global_object);
if (!value.is_empty())
vm().set_last_value({}, value);
if (vm().should_unwind()) {
if (!block.label().is_null() && vm().should_unwind_until(ScopeType::Breakable, block.label()))
vm().stop_unwind();
@ -174,14 +178,18 @@ Value Interpreter::execute_statement(GlobalObject& global_object, const Statemen
}
}
bool did_return = vm().unwind_until() == ScopeType::Function;
if (scope_type == ScopeType::Function) {
bool did_return = vm().unwind_until() == ScopeType::Function;
if (!did_return)
vm().set_last_value({}, js_undefined());
}
if (vm().unwind_until() == scope_type)
vm().unwind(ScopeType::None);
exit_scope(block);
return did_return ? vm().last_value() : js_undefined();
return vm().last_value();
}
LexicalEnvironment* Interpreter::current_environment()