Stage 1 Draft / January 23, 2024

Optional Chaining Assignment

1 Assignment Operators

1.1 Static Semantics: AssignmentTargetType

The syntax-directed operation AssignmentTargetType takes no arguments and returns simple, optional, or invalid. It is defined piecewise over the following productions:

IdentifierReference : Identifier
  1. If this IdentifierReference is contained in strict mode code and StringValue of Identifier is either "eval" or "arguments", return invalid.
  2. Return simple.
 IdentifierReference : yield await CallExpression : CallExpression [ Expression ] CallExpression . IdentifierName CallExpression . PrivateIdentifier MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName SuperProperty MemberExpression . PrivateIdentifier
  1. Return simple.
 OptionalExpression : MemberExpression OptionalChain CallExpression OptionalChain OptionalExpression OptionalChain
  1. Return AssignmentTargetType of OptionalChain.
 OptionalChain : ?. [ Expression ] ?. IdentifierName ?. PrivateIdentifier OptionalChain [ Expression ] OptionalChain . IdentifierName OptionalChain . PrivateIdentifier
  1. Return optional.
 PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return AssignmentTargetType of expr.
 PrimaryExpression : this Literal ArrayLiteral ObjectLiteral FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral CallExpression : CoverCallExpressionAndAsyncArrowHead SuperCall ImportCall CallExpression Arguments CallExpression TemplateLiteral OptionalChain : ?. Arguments ?. TemplateLiteral OptionalChain Arguments OptionalChain TemplateLiteral NewExpression : new NewExpression MemberExpression : MemberExpression TemplateLiteral new MemberExpression Arguments NewTarget : new . target ImportMeta : import . meta LeftHandSideExpression : OptionalExpression UpdateExpression : LeftHandSideExpression ++ LeftHandSideExpression -- ++ UnaryExpression -- UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression typeof UnaryExpression + UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression AwaitExpression ExponentiationExpression : UpdateExpression ** ExponentiationExpression MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression AdditiveExpression : AdditiveExpression + MultiplicativeExpression AdditiveExpression - MultiplicativeExpression ShiftExpression : ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression RelationalExpression : RelationalExpression < ShiftExpression RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression RelationalExpression >= ShiftExpression RelationalExpression instanceof ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in ShiftExpression EqualityExpression : EqualityExpression == RelationalExpression EqualityExpression != RelationalExpression EqualityExpression === RelationalExpression EqualityExpression !== RelationalExpression BitwiseANDExpression : BitwiseANDExpression & EqualityExpression BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression LogicalANDExpression : LogicalANDExpression && BitwiseORExpression LogicalORExpression : LogicalORExpression || LogicalANDExpression CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression ConditionalExpression : ShortCircuitExpression ? AssignmentExpression : AssignmentExpression AssignmentExpression : YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression Expression : Expression , AssignmentExpression
  1. Return invalid.

1.2 Static Semantics: Early Errors

AssignmentExpression : LeftHandSideExpression = AssignmentExpression

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression

1.3 Runtime Semantics: Evaluation

AssignmentExpression : LeftHandSideExpression = AssignmentExpression
  1. If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of LeftHandSideExpression.
    2. If lref is undefined and LeftHandSideExpression is not a ParenthesizedExpression, return undefined.
    3. If IsAnonymousFunctionDefinition(AssignmentExpression) and IsIdentifierRef of LeftHandSideExpression are both true, then
      1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
    4. Else,
      1. Let rref be ? Evaluation of AssignmentExpression.
      2. Let rval be ? GetValue(rref).
    5. If lref is undefined, throw a TypeError exception.
    6. Perform ? PutValue(lref, rval).
    7. Return rval.
  2. Let assignmentPattern be the AssignmentPattern that is covered by LeftHandSideExpression.
  3. Let rref be ? Evaluation of AssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Perform ? DestructuringAssignmentEvaluation of assignmentPattern with argument rval.
  6. Return rval.
 AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. If lref is undefined, then
    1. If LeftHandSideExpression is a ParenthesizedExpression, throw a TypeError exception.
    2. Return undefined.
  3. Let lval be ? GetValue(lref).
  4. Let rref be ? Evaluation of AssignmentExpression.
  5. Let rval be ? GetValue(rref).
  6. Let assignmentOpText be the source text matched by AssignmentOperator.
  7. Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:
    assignmentOpText opText
    **= **
    *= *
    /= /
    %= %
    += +
    -= -
    <<= <<
    >>= >>
    >>>= >>>
    &= &
    ^= ^
    |= |
  8. Let r be ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).
  9. Perform ? PutValue(lref, r).
  10. Return r.
 AssignmentExpression : LeftHandSideExpression &&= AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. If lref is undefined, then
    1. If LeftHandSideExpression is a ParenthesizedExpression, throw a TypeError exception.
    2. Return undefined.
  3. Let lval be ? GetValue(lref).
  4. Let lbool be ToBoolean(lval).
  5. If lbool is false, return lval.
  6. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
  7. Else,
    1. Let rref be ? Evaluation of AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  8. Perform ? PutValue(lref, rval).
  9. Return rval.
 AssignmentExpression : LeftHandSideExpression ||= AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. If lref is undefined, then
    1. If LeftHandSideExpression is a ParenthesizedExpression, throw a TypeError exception.
    2. Return undefined.
  3. Let lval be ? GetValue(lref).
  4. Let lbool be ToBoolean(lval).
  5. If lbool is true, return lval.
  6. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
  7. Else,
    1. Let rref be ? Evaluation of AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  8. Perform ? PutValue(lref, rval).
  9. Return rval.
 AssignmentExpression : LeftHandSideExpression ??= AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. If lref is undefined, then
    1. If LeftHandSideExpression is a ParenthesizedExpression, throw a TypeError exception.
    2. Return undefined.
  3. Let lval be ? GetValue(lref).
  4. If lval is neither undefined nor null, return lval.
  5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
  6. Else,
    1. Let rref be ? Evaluation of AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.

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© 2024 Nicolò Ribaudo

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