typeof Foo['bar'] has strange precedence

[DanielRosenwasser]

Type queries combined with indexed access types currently produce a parse tree that is surprising in its behavior.

One would think that typeof Foo['bar'] would be parsed as typeof (Foo['bar']), which would really be something like typeof Foo.bar.

That's not the case. It's actually parsed as (typeof Foo)['bar'].

Conveniently, it seems that semantically (when type-checking) these are identical, but it seems strange for syntactic consumers. Do we believe this is currently working as intended?

[mhegazy]

It should have the same precedence as in an expression. but this is a breaking change, and so far there is no functional impact of this on the output. marking it as a revisit for now.

[RyanCavanaugh]

Seems too late to change this now.

[DanielRosenwasser]

Worth reconsidering in my opinion #44857

I kind of doubt anyone would be broken by this change.

[fatcerberus]

I remember discovering this a while back and was quite surprised that the relative precedence for typeof vs. [] in a type context was the exact opposite of runtime typeof.

Worth noting though that I’ve seen a lot of examples given out on Gitter and Discord that freely do stuff like typeof Class["prop"], which is how I found out about the odd precedence in the first place. I prefer to add parentheses to minimize confusion, but from what I’ve seen, it is likely to be a breaking change if the precedence is changed now.

[sandersn]

Seems easy enough to make this change and compile tonnes of code to see what the rates of breaks is.

[a-tarasyuk]

TypeQuery is parsed as EntityName. Therefore, currently we can only use the following names in TypeQuery: a (Identifier), a.b (QualifiedName).

TypeScript/src/compiler/parser.ts

Lines 3078 to 3081 in ef9fd97

	function parseTypeQuery(): TypeQueryNode {
	const pos = getNodePos();
	parseExpected(SyntaxKind.TypeOfKeyword);
	return finishNode(factory.createTypeQueryNode(parseEntityName(/*allowReservedWords*/ true)), pos);

@DanielRosenwasser Do we need to parse it as an expression?

/cc @RyanCavanaugh