Stage 2 Draft / July 17, 2023

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 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 throw 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 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 throw 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 Unary Operators

Syntax

UnaryExpression[Yield, Await] : UpdateExpression[?Yield, ?Await] delete UnaryExpression[?Yield, ?Await] void UnaryExpression[?Yield, ?Await] typeof UnaryExpression[?Yield, ?Await] throw UnaryExpression[?Yield, ?Await] [lookahead ∉ ThrowExpressionInvalidPunctuator] + UnaryExpression[?Yield, ?Await] - UnaryExpression[?Yield, ?Await] ~ UnaryExpression[?Yield, ?Await] ! UnaryExpression[?Yield, ?Await] [+Await] AwaitExpression[?Yield] ThrowExpressionInvalidPunctuator : one of , < > <= >= == != === !== + - * / % ** << >> >>> & | ^ && || ?? = += -= *= /= %= **= <<= >>= >>>= &= |= ^= &&= ||= ??= ?

2.2 The throw Operator

2.2.1 Runtime Semantics: Evaluation

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

3 ECMAScript Language: Statements and Declarations

3.1 Expression Statement

Syntax

ExpressionStatement[Yield, Await] : [lookahead ∉ { {, function, async [no LineTerminator here] function, class, let [ }] Expression[+In, ?Yield, ?Await] ; [lookahead ∉ { {, function, async [no LineTerminator here] function, class, let [, throw }] Expression[+In, ?Yield, ?Await] ; Note

An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. An ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with async function because that would make it ambiguous with an AsyncFunctionDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern. An ExpressionStatement cannot start with throw because that would make it ambiguous with a ThrowStatement.

A Copyright & Software License

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© 2023 Ron Buckton, Ecma International

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