The expression subset

A query lambda is not arbitrary JavaScript. It is an expression-bodied arrow over a small, closed grammar — the set of forms that have a defined meaning across every provider. Anything outside the subset is rejected with a coded, located diagnostic, and you get the same message in your editor, in ESLint or oxlint, and at build time.

Allowed

• Parameters: plain identifiers, or object-destructuring with identifier shorthand (({ id, name }) => …).
• Member access u.name and index access u.items[0], including optional chaining (u.a?.b).
• Calls, including method calls (u.name.startsWith("A")) and nested lambdas for some / every (u.tags.some((t) => t.startsWith("a"))).
• Binary, logical, unary, and ternary operators; template literals; object and array literals.
• Captured free variables (minAge) and safelisted globals (Math, Date, JSON, …).

TypeScript-only syntax is stripped, not rejected: as, satisfies, non-null !, and type arguments all disappear before the tree is built.

Rejected

Out-of-subset syntax produces an error with a stable code (see the error reference):

Form	Code
Block-bodied arrow { … }	R1101
Assignment / update operators	R1102
Loose equality == / !=	R1103
this	R1104
new	R1105
await / yield	R1106
Non-arrow function expressions	R1107
Tagged templates	R1108
Regex literals in the body	R1109

Loose equality is rejected because == has no single cross-provider meaning; the ESLint rule autofixes it to ===. A regex literal is rejected inside the body — hoist it to a const above the query and capture it instead.

Why a closed grammar

A provider promises to translate what it receives. That promise is only possible over a finite set of node kinds: with a closed grammar, "can this be translated?" is a total function, not an open-world search. The one open dimension is which functions a call targets, and each provider decides that explicitly — translating it, folding it when its arguments are constant, or rejecting it with a located error that names the provider and the call.