TypeScript default overloads treated as multiple default exports

[AndrewLeedham]

The following example causes 3 error Multiple default exports import/export errors, but there is only technically 1 default export the other 2 are overloads.

export default function foo(param: string): boolean;
export default function foo(param: string, param1: number): boolean;
export default function foo(param: string, param1?: number): boolean {
  return param && param1;
}

There seems to be some code for checking overloads: https://github.com/benmosher/eslint-plugin-import/blob/master/src/rules/export.js#L37-L40 but this does not seem to catch this case. I added some debug logs in and the rough contents of namespace is:

root: {
  default: [
    {
       type: 'ExportDefaultDeclaration'
    },
    {
       type: 'ExportDefaultDeclaration'
    },
    {
       type: 'ExportDefaultDeclaration'
    }
  ]
}

I am running this in the following environment:

• eslint: 5.16.0
• eslint-plugin-import: 2.19.1
• eslint-import-resolver-typescript: 2.0.0
• typescript: 3.7.2
• @typescript-eslint/parser: 2.13.0

Related:

• no-duplicate-default-export #22 (comment)
• import/export: false positives for typescript overloads #1357

[benjie]

A workaround is:

function foo(param: string): boolean;
function foo(param: string, param1: number): boolean;
function foo(param: string, param1?: number): boolean {
  return param && param1;
}

export default foo;

[eyedean]

@benjie sure, that unblocks the case. But, ideally, it should be able to detect it's an overload.

PS. Might be obvious, but just in case for beginners, for non-default exports you similarly do:

function foo(param: string): boolean;
function foo(param: string, param1: number): boolean;
function foo(param: string, param1?: number): boolean {
  return param && param1;
}

export { foo };

[AverageHelper]

Can confirm this issue is still present. I too think that the plugin should be able to detect that this case is an overload, and not actually a case of multiple default exports.

[azhaochen]

@benjie sure, that unblocks the case. But, ideally, it should be able to detect it's an overload.

PS. Might be obvious, but just in case for beginners, for non-default exports you similarly do:

function foo(param: string): boolean;
function foo(param: string, param1: number): boolean;
function foo(param: string, param1?: number): boolean {
  return param && param1;
}

export { foo };

export { foo } should put in the first line, or it will report error 'An export declaration can only be used at the top level of a namespace or module.ts(1233)'

[jordanbtucker]

Just checking to see if there's an ETA for this fix to land in a release.

[ljharb]

No, there’s no timeline for releases.

[TomasHubelbauer]

export default function test(str: string): void;
export default function test(num: number, str: string): void;
export default function test(arg: string | number, str?: string) {
  console.log(arg, str);
}

Output:

Multiple default exports. eslint(import/export)

Looks like this is still present in some form or another. Or is my overload incorrect? There are no errors when pasting the above snippet to the TypeScript Playground verbatim.

[ljharb]

@TomasHubelbauer what version of the plugin are you using?

[TomasHubelbauer]

2.26.0_eslint@8.40.0 - I am using it through eslint-config-next 13.4.1

[ljharb]

@TomasHubelbauer the fix for this was in v2.27.5, so that's your problem.

[TomasHubelbauer]

I see! Thanks!