Stage 2 Draft / January 20, 2024

ECMAScript 'throw' expressions

Introduction

This proposal defines new syntax to throw exceptions from within an expression context.

See the proposal repository for background material and discussion.

1 Syntax-Directed Operations

In addition to those defined in this section, specialized syntax-directed operations are defined throughout this specification.

1.1 Function Name Inference

1.1.1 Static Semantics: IsFunctionDefinition

The syntax-directed operation IsFunctionDefinition takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return IsFunctionDefinition of expr.
 PrimaryExpression : this IdentifierReference Literal ArrayLiteral ObjectLiteral RegularExpressionLiteral TemplateLiteral ThrowExpression : throw Expression MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName MemberExpression TemplateLiteral SuperProperty MetaProperty new MemberExpression Arguments MemberExpression . PrivateIdentifier NewExpression : new NewExpression LeftHandSideExpression : CallExpression 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 LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression Expression : Expression , AssignmentExpression
  1. Return false.
 AssignmentExpression : ArrowFunction AsyncArrowFunction FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody } ClassExpression : class BindingIdentifieropt ClassTail
  1. Return true.

1.2 Miscellaneous

These operations are used in multiple places throughout the specification.

1.2.1 Static Semantics: AssignmentTargetType

The syntax-directed operation AssignmentTargetType takes no arguments and returns simple 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.
 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 ThrowExpression : throw Expression CallExpression : CoverCallExpressionAndAsyncArrowHead SuperCall ImportCall CallExpression Arguments CallExpression 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.

2 ECMAScript Language: Expressions

2.1 Primary Expression

Syntax

PrimaryExpression[Yield, Await] : this IdentifierReference[?Yield, ?Await] Literal ArrayLiteral[?Yield, ?Await] ObjectLiteral[?Yield, ?Await] FunctionExpression ClassExpression[?Yield, ?Await] GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral[?Yield, ?Await, ~Tagged] CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] CoverParenthesizedExpressionAndArrowParameterList[Yield, Await] : ( Expression[+In, ?Yield, ?Await] ) ( NestedExpression[?Yield, ?Await] ) ( Expression[+In, ?Yield, ?Await] , ) ( ) ( ... BindingIdentifier[?Yield, ?Await] ) ( ... BindingPattern[?Yield, ?Await] ) ( Expression[+In, ?Yield, ?Await] , ... BindingIdentifier[?Yield, ?Await] ) ( Expression[+In, ?Yield, ?Await] , ... BindingPattern[?Yield, ?Await] )

Supplemental Syntax

When processing an instance of the production
PrimaryExpression[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression[Yield, Await] : ( Expression[+In, ?Yield, ?Await] ) ( NestedExpression[?Yield, ?Await] ) NestedExpression[Yield, Await] : Expression[+In, ?Yield, ?Await] ThrowExpression[?Yield, ?Await]

2.1.1 The Grouping Operator

2.1.1.1 Static Semantics: Early Errors

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

2.1.1.2 Runtime Semantics: Evaluation

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ? Evaluation of expr.
 ParenthesizedExpression : ( Expression ) ParenthesizedExpression : ( NestedExpression )
  1. Return ? Evaluation of ExpressionNestedExpression. This may be of type Reference.
 Note

This algorithm does not apply GetValue to Evaluation of ExpressionNestedExpression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.

2.1.2 The throw Operator

Syntax

ThrowExpression[Yield, Await] : throw Expression[?In, ?Yield, ?Await]

2.1.2.1 Runtime Semantics: Evaluation

ThrowExpression : throw Expression
  1. Let exprRef be ? Evaluation of Expression.
  2. Let exprValue be ? GetValue(exprRef).
  3. Return ThrowCompletion(exprValue).

2.2 ECMAScript Language: Functions and Classes

2.2.1 Tail Position Calls

2.2.1.1 Static Semantics: HasCallInTailPosition

The syntax-directed operation HasCallInTailPosition takes argument call (a CallExpression Parse Node, a MemberExpression Parse Node, or an OptionalChain Parse Node) and returns a Boolean.

Note 1

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.

 Note 2

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

It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let has be HasCallInTailPosition of StatementList with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of StatementListItem with argument call.
 FunctionStatementList : [empty] StatementListItem : Declaration Statement : VariableStatement EmptyStatement ExpressionStatement ContinueStatement BreakStatement ThrowStatement DebuggerStatement Block : { } ReturnStatement : return ; LabelledItem : FunctionDeclaration ForInOfStatement : for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement CaseBlock : { }
  1. Return false.
 IfStatement : if ( Expression ) Statement else Statement
  1. Let has be HasCallInTailPosition of the first Statement with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of the second Statement with argument call.
 IfStatement : if ( Expression ) Statement DoWhileStatement : do Statement while ( Expression ) ; WhileStatement : while ( Expression ) Statement ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement WithStatement : with ( Expression ) Statement
  1. Return HasCallInTailPosition of Statement with argument call.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return HasCallInTailPosition of LabelledItem with argument call.
 ReturnStatement : return Expression ;
  1. Return HasCallInTailPosition of Expression with argument call.
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return HasCallInTailPosition of CaseBlock with argument call.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. Let has be false.
  2. If the first CaseClauses is present, let has be HasCallInTailPosition of the first CaseClauses with argument call.
  3. If has is true, return true.
  4. Let has be HasCallInTailPosition of DefaultClause with argument call.
  5. If has is true, return true.
  6. If the second CaseClauses is present, let has be HasCallInTailPosition of the second CaseClauses with argument call.
  7. Return has.
 CaseClauses : CaseClauses CaseClause
  1. Let has be HasCallInTailPosition of CaseClauses with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of CaseClause with argument call.
 CaseClause : case Expression : StatementListopt DefaultClause : default : StatementListopt
  1. If StatementList is present, return HasCallInTailPosition of StatementList with argument call.
  2. Return false.
 TryStatement : try Block Catch
  1. Return HasCallInTailPosition of Catch with argument call.
 TryStatement : try Block Finally try Block Catch Finally
  1. Return HasCallInTailPosition of Finally with argument call.
 Catch : catch ( CatchParameter ) Block
  1. Return HasCallInTailPosition of Block with argument call.
 AssignmentExpression : YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression 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 RelationalExpression instanceof ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in ShiftExpression ShiftExpression : ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression AdditiveExpression : AdditiveExpression + MultiplicativeExpression AdditiveExpression - MultiplicativeExpression MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression ExponentiationExpression : UpdateExpression ** ExponentiationExpression UpdateExpression : LeftHandSideExpression ++ LeftHandSideExpression -- ++ UnaryExpression -- UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression typeof UnaryExpression + UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression AwaitExpression CallExpression : SuperCall CallExpression [ Expression ] CallExpression . IdentifierName CallExpression . PrivateIdentifier NewExpression : new NewExpression MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName SuperProperty MetaProperty new MemberExpression Arguments MemberExpression . PrivateIdentifier NestedExpression : ThrowExpression 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 CallExpression Arguments CallExpression TemplateLiteral
  1. If this CallExpression is call, return true.
  2. Return false.
 OptionalExpression : MemberExpression OptionalChain CallExpression OptionalChain OptionalExpression OptionalChain
  1. Return HasCallInTailPosition of OptionalChain with argument call.
 OptionalChain : ?. [ Expression ] ?. IdentifierName ?. PrivateIdentifier OptionalChain [ Expression ] OptionalChain . IdentifierName OptionalChain . PrivateIdentifier
  1. Return false.
 OptionalChain : ?. Arguments OptionalChain Arguments
  1. If this OptionalChain is call, return true.
  2. Return false.
 MemberExpression : MemberExpression TemplateLiteral
  1. If this MemberExpression is call, return true.
  2. Return false.
 PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return HasCallInTailPosition of expr with argument call.
 ParenthesizedExpression : ( Expression )
  1. Return HasCallInTailPosition of Expression with argument call.

A Copyright & Software License

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© 2024 Ron Buckton, Ecma International

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