import-js/eslint-plugin-import

False positive: "Multiple default exports" with merged declarations in Typescript

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#1,549 opened on Nov 30, 2019

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bughelp wantedtypescript

Description

In Typescript, a class and an interface with the same name are merged into one type. If I want to export that type as default, I have to export default both the class and the interface.

This is incorrectly flagged with Multiple default exports.

Code:

export default interface Foo {
  a: number
}
export default class Foo {}

Actual:

1:1  error  Multiple default exports  import/export
4:1  error  Multiple default exports  import/export

Expected: No linting error.

Versions:

  • Typescript 3.7.2
  • eslint-plugin-import 2.18.2

Contributor guide

Tech stack
javascripttypescript
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.
half day
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 knowledgeeslint plugin import architecture
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
The issue is a false positive in the 'export' rule of eslint plugin import. The rule is likely implemented in src/rules/export.js. To fix, need to detect when multiple default exports are actually TypeScript declaration merging of the same name. Look for cases where both exports are of the same identifier and are a class and an interface (or other mergeable types). The plugin may need to use TypeScript's type checker or just check for the pattern where the exported names are identical. Consider referencing TypeScript's declaration merging documentation.