Microsoft/TypeScript

typeof Foo['bar'] has strange precedence

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#19,707 opened on Nov 3, 2017

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 (6 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
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Description

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?

Contributor guide

Tech stack
typescript
Domain
toolingdeveloper experience
Issue type
research
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript type systemcompiler parser basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the parser implementation in src/compiler/parser.ts to see how typeof and indexed access types are parsed. Review the relevant grammar rules and check if the current behavior is deliberate. If not, propose a fix to change the parse tree. Additionally, check if any existing tests cover this precedence.