TS2834: fxp.d.ts imports './pem' without extension under NodeNext moduleResolution

[tomquist]

• Are you running the latest version?
• Have you included sample input, output, error, and expected output?
• Have you checked if you are using correct configuration?
• Did you try online tool? (not applicable — TypeScript resolution issue, not runtime parse output)
• Have you checked the docs for helpful APIs and examples?

Description

With moduleResolution set to "node16" or "nodenext" (TypeScript 5.x), the package’s published ESM types entry (package.json → exports["."].import.types → ./src/fxp.d.ts) fail to type-check because src/fxp.d.ts uses a relative import without a .js extension:

import type { Expression, ReadonlyMatcher } from './pem';

TypeScript reports TS2834 (“Relative import paths need explicit file extensions in ECMAScript imports when --moduleResolution is node16 or nodenext”).

This affects consumers that type-check dependencies (e.g. skipLibCheck: false) and pull types through fast-xml-parser as a transitive dependency (for example via AWS SDK for JavaScript v3’s @aws-sdk/xml-builder, which depends on fast-xml-parser — see aws-sdk-js-v3 v3.1014.0 changelog mentioning the xml-builder / fast-xml-parser bump).

Suggested fix (either or both):

1. In src/fxp.d.ts, use a NodeNext-valid specifier, e.g. from './pem.js' (TypeScript resolves this to pem.d.ts).
2. Or point exports["."].import.types at declaration files that already satisfy NodeNext rules (e.g. generated .d.ts with correct extensions), if you prefer not to hand-edit src/*.d.ts.

Input

TypeScript project using NodeNext resolution (minimal tsconfig.json excerpt):

{
  "compilerOptions": {
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "strict": true,
    "skipLibCheck": false
  }
}

Code

Any dependency path that causes the compiler to load the ESM types for the main entry, e.g.:

// package.json (consumer)
{
  "dependencies": {
    "fast-xml-parser": "5.5.8"
  }
}

Then run the compiler; no application import of fast-xml-parser is required if another package’s types pull in fxp.d.ts (e.g. via @aws-sdk/xml-builder).

Alternatively, a minimal direct repro:

// Any .ts file
import type { XMLParser } from "fast-xml-parser";
type _ = XMLParser;

Output

node_modules/fast-xml-parser/src/fxp.d.ts(1,50): error TS2834: Relative import paths need explicit file extensions in ECMAScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Consider adding an extension to the import path.

expected data

• tsc should succeed with moduleResolution: "NodeNext" and skipLibCheck: false when resolving types for fast-xml-parser@5.5.8 (and future patch releases), without requiring consumers to patch node_modules or enable skipLibCheck.

Environment: fast-xml-parser@5.5.8, TypeScript 5.9 (or any 5.x with TS2834 for extensionless relative imports in .d.ts).

---

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[loucadufault]

workaround (as highlighted in the post) is to configure TS to set skipLibCheck, e.g.:

tsconfig.json

{
  "compilerOptions": {
    "skipLibCheck": true
  }
}

[benasher44]

It's not a great workaround, since you then lose the ability for TypeScript to tell you when your dependencies have mismatched types for all of your dependencies.

[amitguptagwl]

I had combined them in a single file. Does it still cause issue?

[tomquist]

I can confirm that this is fixed in 5.5.9