in inside an array literal in a for initializer is a syntax error (for (let i = 0, a = [k in o]; …)) #65311

🔎 Search Terms

in operator for loop initializer array literal, TS1005 ',' expected, DisallowInContext, for head in expression, for-of destructuring default in

🕗 Version & Regression Information

This is the behavior in every version I tried: 5.9.3, 6.0.3, 7.0.2, 7.1.0-dev.20261103.1 (typescript@next) and main e2bc2bc. Not a regression.

⏯ Playground Link

No response

💻 Code

const o = { a: 1 }, k = "a";
for (let i = 0, has = [k in o]; i < 1; i++) {}
for (let i = 0, has = { v: k in o }; i < 1; i++) {}
for (let i = 0, has = (k in o); i < 1; i++) {}
let x;
for ([x = "x" in {}] of [[]]) {}

🙁 Actual behavior

tsc --noEmit --target esnext --module esnext --pretty false t.ts:

t.ts(2,26): error TS1005: ',' expected.
t.ts(2,30): error TS1005: ')' expected.
t.ts(2,43): error TS1005: ';' expected.
t.ts(6,15): error TS1005: ',' expected.

Lines 2 and 6 fail. The same k in o inside an object literal (line 3) or parentheses (line 4) is accepted, as it is in call arguments and template substitutions.

🙂 Expected behavior

No errors. The [~In] of the for head (for ( LexicalDeclaration[~In] Expression[+In]opt ; …), for ( Expression[~In]opt ; …), https://tc39.es/ecma262/#sec-for-statement) does not reach into array literals: ElementList : AssignmentExpression[+In] (https://tc39.es/ecma262/#sec-array-initializer), the same as for object literal property values. In the for-of case the array literal is an ArrayAssignmentPattern, whose Initializer[+In] also allows in. node accepts the file; for (let i = 0, has = [k in o]; i < 1; i++) console.log(has); prints [ true ].

Additional information about the issue

parseArrayLiteralExpression in internal/parser/parser.go parses its elements in the caller's context, so NodeFlagsDisallowInContext from the for initializer is still set. parseArgumentExpression already clears it for call arguments. Clearing it for the element list fixes both lines:

elements := p.doInContext(ast.NodeFlagsDisallowInContext, false, func(p *Parser) *ast.NodeList {
	return p.parseDelimitedList(PCArrayLiteralMembers, (*Parser).parseArgumentOrArrayLiteralElement)
})

With that change on main, the code above has no errors, for (var q = "a" in o; ;) break; is still rejected, and go test ./internal/parser/ passes.