VIP-41: TC39 throw expressions

Status: accepted by Will, 2026-11-02.

Will's words

"Refer to tc39 spec for answer"

Recorded in WILL-ANSWERS.md, 2026-11-02. As with VIP-2's do expressions, adopt the proposal text exactly for grammar, precedence and positions requiring parentheses.

Decision

The normative reference is the vendored TC39 throw-expressions specification, pinned to tc39/proposal-throw-expressions commit 78b887751db4e380870a0a6bfc47319c50aa1404 (2024-01-19). Appendix A imports its unary production, invalid-punctuator lookahead and expression-statement restriction.

A throw expression takes a unary operand and is itself a unary expression. Parentheses around the operand allow a compound value: x ?? throw (a + b). Parentheses around the throw allow it before another operator: (throw e) + b. A following comma is forbidden too: write f((throw e), b) or [a, throw e]. The restriction is lexical: in and instanceof are absent from the invalid-punctuator list. TypeScript's as and satisfies are also absent and retain their ordinary grammar. A statement starting with throw remains a throw statement, with its full expression operand and no-line-terminator restriction. An expression-position throw has no such line-terminator restriction. ECMAScript automatic semicolon insertion still applies.

Throw expressions remain supported; no migration to do { throw (operand); } is required. Existing do blocks remain valid.

The existing EK1010 identifies a throw-expression lookahead violation. Native ECMAScript syntax restrictions keep their existing diagnostics. No new code is allocated outside the EK registry.

Existing placement and match-arm rules

Where a throw is allowed at all is not this VIP's decision. VIP-4's no-throw rule (EK0401) decides that. In effect code a throw fails the Effect, exactly as the corresponding throw statement does. Grammar acceptance does not grant permission to throw from a plain function or from eager parameter initialization.

VIP-7's direct match-arm body has its separate "throw" Expression production: default: throw a + b; throws the full operand, as a throw statement does. A throw expression nested within an arm still follows VIP-41.