Stage 4 Draft / July 21, 2020

Logical Assignment Operators

1 Scope

This is the spec text of the Logical Assignment proposal in ECMAScript.

Each Logical Assignment operator embraces the short-circuiting semantics of the respective Binary Logical operator. If the operator short circuits, then the [[Set]] operation is skipped.

The new LogicalAssignmentPunctuator production defined in Punctuators section is used only to highlight the new additions. When merged with the main ecma262 spec, the new punctuators should just be added to OtherPunctuator.

2 Punctuators (11.7)

Syntax

Punctuator::LogicalAssignmentPunctuator OptionalChainingPunctuator OtherPunctuator LogicalAssignmentPunctuator::one of&&=||=??= OptionalChainingPunctuator::?.[lookahead ∉ DecimalDigit] OtherPunctuator::one of{()[]....;,<><=>===!====!==+-*%**++--<<>>>>>&|^!~&&||???:=+=-=*=%=**=<<=>>=>>>=&=|=^==> DivPunctuator::/ /= RightBracePunctuator::}

3 Assignment Operators (12.15)

Syntax

AssignmentExpression[In, Yield, Await]:ConditionalExpression[?In, ?Yield, ?Await] [+Yield]YieldExpression[?In, ?Await] ArrowFunction[?In, ?Yield, ?Await] AsyncArrowFunction[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]=AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]AssignmentOperatorAssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]&&=AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]||=AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]??=AssignmentExpression[?In, ?Yield, ?Await] AssignmentOperator:one of*=/=%=+=-=<<=>>=>>>=&=^=|=**=

3.1 Static Semantics: Early Errors

AssignmentExpression:LeftHandSideExpression=AssignmentExpression AssignmentExpression:LeftHandSideExpressionAssignmentOperatorAssignmentExpression LeftHandSideExpression&&=AssignmentExpression LeftHandSideExpression||=AssignmentExpression LeftHandSideExpression??=AssignmentExpression

3.2 Static Semantics: IsFunctionDefinition

AssignmentExpression:ArrowFunction AsyncArrowFunction
  1. Return true.
 AssignmentExpression:YieldExpression LeftHandSideExpression=AssignmentExpression LeftHandSideExpressionAssignmentOperatorAssignmentExpression LeftHandSideExpression&&=AssignmentExpression LeftHandSideExpression||=AssignmentExpression LeftHandSideExpression??=AssignmentExpression
  1. Return false.

3.3 Static Semantics: AssignmentTargetType

AssignmentExpression:YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression=AssignmentExpression LeftHandSideExpressionAssignmentOperatorAssignmentExpression LeftHandSideExpression&&=AssignmentExpression LeftHandSideExpression||=AssignmentExpression LeftHandSideExpression??=AssignmentExpression
  1. Return invalid.

3.4 Runtime Semantics: Evaluation

AssignmentExpression:LeftHandSideExpression=AssignmentExpression
  1. If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be the result of evaluating LeftHandSideExpression.
    2. ReturnIfAbrupt(lref).
    3. If IsAnonymousFunctionDefinition(AssignmentExpression) and IsIdentifierRef of LeftHandSideExpression are both true, then
      1. Let rval be NamedEvaluation of AssignmentExpression with argument GetReferencedName(lref).
    4. Else,
      1. Let rref be the result of evaluating AssignmentExpression.
      2. Let rval be ? GetValue(rref).
    5. Perform ? PutValue(lref, rval).
    6. Return rval.
  2. Let assignmentPattern be the AssignmentPattern that is covered by LeftHandSideExpression.
  3. Let rref be the result of evaluating AssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Perform ? DestructuringAssignmentEvaluation of assignmentPattern using rval as the argument.
  6. Return rval.
 AssignmentExpression:LeftHandSideExpressionAssignmentOperatorAssignmentExpression
  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluating AssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Let assignmentOpText be the source text matched by AssignmentOperator.
  6. Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:
    assignmentOpText opText
    **= **
    *= *
    /= /
    %= %
    += +
    -= -
    <<= <<
    >>= >>
    >>>= >>>
    &= &
    ^= ^
    |= |
  7. Let r be ApplyStringOrNumericBinaryOperator(lval, opText, rval).
  8. Perform ? PutValue(lref, r).
  9. Return r.
 AssignmentExpression:LeftHandSideExpression&&=AssignmentExpression
  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ! ToBoolean(lval).
  4. If lbool is false, return lval.
  5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be NamedEvaluation of AssignmentExpression with argument GetReferencedName(lref).
  6. Else,
    1. Let rref be the result of evaluating AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.
 AssignmentExpression:LeftHandSideExpression||=AssignmentExpression
  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ! ToBoolean(lval).
  4. If lbool is true, return lval.
  5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be NamedEvaluation of AssignmentExpression with argument GetReferencedName(lref).
  6. Else,
    1. Let rref be the result of evaluating AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.
 AssignmentExpression:LeftHandSideExpression??=AssignmentExpression
  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. If lval is neither undefined nor null, return lval.
  4. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be NamedEvaluation of AssignmentExpression with argument GetReferencedName(lref).
  5. Else,
    1. Let rref be the result of evaluating AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  6. Perform ? PutValue(lref, rval).
  7. Return rval.
 Note

When this expression occurs within strict mode code, it is a runtime error if lref in step 1.e, 2, 2, 2, 2 is an unresolvable reference. If it is, a ReferenceError exception is thrown. Additionally, it is a runtime error if the lref in step 8, 7, 7, 6 is a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, or to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.

4 Static Semantics: HasCallInTailPosition (14.9.2)

With parameter call.

Note

call is a Parse Node that represents a specific range of source text. When the following algorithms compare call to another Parse Node, it is a test of whether they represent the same source text.

4.1 Expression Rules (14.9.2.2)

Note

A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. Function calls cannot return reference values, so such a GetValue operation will always return the same value as the actual function call result.

 AssignmentExpression:YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression=AssignmentExpression LeftHandSideExpressionAssignmentOperatorAssignmentExpression LeftHandSideExpression&&=AssignmentExpression LeftHandSideExpression||=AssignmentExpression LeftHandSideExpression??=AssignmentExpression BitwiseANDExpression:BitwiseANDExpression&EqualityExpression BitwiseXORExpression:BitwiseXORExpression^BitwiseANDExpression BitwiseORExpression:BitwiseORExpression|BitwiseXORExpression EqualityExpression:EqualityExpression==RelationalExpression EqualityExpression!=RelationalExpression EqualityExpression===RelationalExpression EqualityExpression!==RelationalExpression RelationalExpression:RelationalExpression<ShiftExpression RelationalExpression>ShiftExpression RelationalExpression<=ShiftExpression RelationalExpression>=ShiftExpression RelationalExpressioninstanceofShiftExpression RelationalExpressioninShiftExpression ShiftExpression:ShiftExpression<<AdditiveExpression ShiftExpression>>AdditiveExpression ShiftExpression>>>AdditiveExpression AdditiveExpression:AdditiveExpression+MultiplicativeExpression AdditiveExpression-MultiplicativeExpression MultiplicativeExpression:MultiplicativeExpressionMultiplicativeOperatorExponentiationExpression ExponentiationExpression:UpdateExpression**ExponentiationExpression UpdateExpression:LeftHandSideExpression++ LeftHandSideExpression-- ++UnaryExpression --UnaryExpression UnaryExpression:deleteUnaryExpression voidUnaryExpression typeofUnaryExpression +UnaryExpression -UnaryExpression ~UnaryExpression !UnaryExpression AwaitExpression CallExpression:SuperCall CallExpression[Expression] CallExpression.IdentifierName NewExpression:newNewExpression MemberExpression:MemberExpression[Expression] MemberExpression.IdentifierName SuperProperty MetaProperty newMemberExpressionArguments PrimaryExpression:this IdentifierReference Literal ArrayLiteral ObjectLiteral FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral
  1. Return false.
 Expression:AssignmentExpression Expression,AssignmentExpression
  1. Return HasCallInTailPosition of AssignmentExpression with argument call.
 ConditionalExpression:ShortCircuitExpression?AssignmentExpression:AssignmentExpression
  1. Let has be HasCallInTailPosition of the first AssignmentExpression with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of the second AssignmentExpression with argument call.
 LogicalANDExpression:LogicalANDExpression&&BitwiseORExpression
  1. Return HasCallInTailPosition of BitwiseORExpression with argument call.
 LogicalORExpression:LogicalORExpression||LogicalANDExpression
  1. Return HasCallInTailPosition of LogicalANDExpression with argument call.
 CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
  1. Return HasCallInTailPosition of BitwiseORExpression with argument call.
 CallExpression:CoverCallExpressionAndAsyncArrowHead CallExpressionArguments CallExpressionTemplateLiteral
  1. If this CallExpression is call, return true.
  2. Return false.
 OptionalExpression:MemberExpressionOptionalChain CallExpressionOptionalChain OptionalExpressionOptionalChain
  1. Return HasCallInTailPosition of OptionalChain with argument call.
 OptionalChain:?.[Expression] ?.IdentifierName OptionalChain[Expression] OptionalChain.IdentifierName
  1. Return false.
 OptionalChain:?.Arguments OptionalChainArguments
  1. If this OptionalChain is call, return true.
  2. Return false.
 MemberExpression:MemberExpressionTemplateLiteral
  1. If this MemberExpression is call, return true.
  2. Return false.
 PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
  2. Return HasCallInTailPosition of expr with argument call.
 ParenthesizedExpression:(Expression)
  1. Return HasCallInTailPosition of Expression with argument call.

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