[ESNext] Implement logical assignment operators
https://bugs.webkit.org/show_bug.cgi?id=209716
Reviewed by Ross Kirsling.
JSTests:
- stress/logical-assignment-operator-and.js: Added.
- stress/logical-assignment-operator-nullish.js: Added.
- stress/logical-assignment-operator-or.js: Added.
- test262/config.yaml:
- test262/expectations.yaml:
Right now, test262 expects an early error to be thrown if the lhs is not simple, which does
not match what we do with other read-modify assignment operators. This is likely to change
in the future to match existing behavior (throw a ReferenceError) [1].
[1]: <https://github.com/tc39/ecma262/issues/257#issuecomment-502878708>
Source/JavaScriptCore:
Implement the logical assignment operators proposal, which is now Stage 3. It introduces
three new assignment operators which will only store the result of the rhs in the lhs if the
lhs meets the given condition:
??=, for if the lhs is nullish (null or undefined)
||=, for if the lhs is falsy
&&=, for if the lhs is truthy
This short circuiting can be beneficial as it can avoid a redundant store when used in the
common JavaScript programming pattern of "defaulting" a parameter.
function foo(x) {
}
If x is a truthy value, it would result in the rhs x being stored back into the lhs x.
In some situations, this can have negative unintended side-effects, such as for innerHTML.
Logical assignment operators, however, are defined such that they only store if the rhs is
to actually be needed/used, skipping the redundant store and simply returning lhs otherwise.
In the case of readonly references, this means that an error is only thrown when the
assignment occurs, meaning that if the lhs already satisfies the condition it will be used
and returned with no error.
- parser/ParserTokens.h:
- parser/Lexer.cpp:
(JSC::Lexer<T>::lexWithoutClearingLineTerminator):
(JSC::Parser<LexerType>::parseAssignmentExpression):
(JSC::ASTBuilder::makeAssignNode):
- parser/Nodes.h:
- parser/NodeConstructors.h:
(JSC::ShortCircuitReadModifyResolveNode::ShortCircuitReadModifyResolveNode): Added.
(JSC::ShortCircuitReadModifyBracketNode::ShortCircuitReadModifyBracketNode): Added.
(JSC::ShortCircuitReadModifyDotNode::ShortCircuitReadModifyDotNode): Added.
- bytecompiler/NodesCodegen.cpp:
(JSC::emitShortCircuitAssignment): Added.
(JSC::ShortCircuitReadModifyResolveNode::emitBytecode): Added.
(JSC::ShortCircuitReadModifyDotNode::emitBytecode): Added.
(JSC::ShortCircuitReadModifyBracketNode::emitBytecode): Added.
Add a useLogicalAssignmentOperators setting for controlling this feature.
Tools:
- Scripts/run-jsc-stress-tests:
