Microsoft/TypeScript

ImportType attributes can include JS expressions

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#62,179 opened on Aug 3, 2025

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 (1 comment) (1 reaction) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: ES ModulesHelp Wanted

Description

🔎 Search Terms

importtype, import type parsing

🕗 Version & Regression Information

  • This changed between versions 4.6.4 and 4.7.4 (with the old assert keyword)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20250802#code/CYUwxgNghgTiAEBbA9sArhBAiA7lALlgNwBQ+AngA4ID68AvPBdcgGbwCWilyM+AFLgJYANPADeJePCgBnWSD4AuCVOkyV-fgEoGAPlXr1YZADtZyTADoIyAOaCcAC3LwwUU-HLI08HDA58BAArWXgnRRAAfhiorG1SIwBfbR0RNSSSFNIgA

💻 Code

declare module "wat";
type _ = typeof import("wat", {
  assert: {
    a: (() => {
      console.log("why can you write js here????");
    })(),
  }
});

🙁 Actual behavior

parser accepts arbitrary JS expressions inside a type expression

🙂 Expected behavior

parser only allows string literal values or arbitrary type values

Additional information about the issue

No response

Contributor guide

Tech stack
typescript
Domain
toolingdeveloper experience
Issue type
bug
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript basicscompiler/parser concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
The issue is about the parser accepting arbitrary JS expressions in ImportType attributes (like the 'assert' block). Likely the affected code is in the TypeScript parser handling import expressions, specifically the parsing of import type attributes. Look at the parser's handling of ImportType (e.g., in parser.ts or checker.ts) and ensure that only string literal types or type expressions are allowed. There are no linked PRs; this is a fresh bug.