TypeScript native / TS 6+ (Next 16) – TS1540 on declare module slugify in slugify.d.ts

[fringe4life]

Hi, thanks team slugify – i'm using it in a Next.js 16 monorepo with the new TypeScript “native” compiler, and am running into a hard TS error from the bundled .d.ts.

Error

With slugify@1.6.6 and the native TypeScript compiler (via @typescript/native-preview), I consistently get:
ts
../../node_modules/.bun/slugify@1.6.6/node_modules/slugify/slugify.d.ts:1:16 - error TS1540: A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.
1 declare module slugify

This happens even with skipLibCheck: true – it appears to be treated as a syntax / deprecation error in the newer compiler rather than a regular lib type error.

Minimal reproduction

Dependencies (simplified):

{
"dependencies": {
"slugify": "^1.6.6"
},
"devDependencies": {
"@typescript/native-preview": "^7.0.0-dev.20260225.1"
}
}

tsconfig.json (Next.js 16-style):

{
"compilerOptions": {
"target": "es2022",
"lib": ["dom", "dom.iterable", "esnext"],
"types": ["node", "bun"],
"alwaysStrict": true,
"allowJs": true,
"skipLibCheck": true,
"strict": true,
"noEmit": true,
"esModuleInterop": true,
"module": "esnext",
"moduleResolution": "bundler",
"resolveJsonModule": true,
"isolatedModules": true,
"jsx": "react-jsx"
},
"include": [
"next-env.d.ts",
"/.ts",
"/.tsx"
],
"exclude": ["node_modules"]
}

Code:

import slugify from "slugify";
const s = slugify("Example title");

Command (from the Next.js app package):

tsgo --noEmit --skipLibCheck --project tsconfig.json
tsgo internally uses @typescript/native-preview / the TS 6+ compiler

This fails with TS1540 pointing at slugify.d.ts line 1.

Context

• TypeScript 6+ (and the “native” compiler used by Next 16 / tsgo) has tightened rules around module vs namespace for declaration files.
• The declaration in slugify.d.ts still uses declare module slugify { ... }, which now triggers TS1540.
• skipLibCheck doesn’t avoid it, since this is treated as an invalid declaration form rather than a type‑checking issue.

There are a couple of related existing issues:

• Module NodeNext : TS2349: This expression is not callable #185 “Module NodeNext : TS2349: This expression is not callable”
• "not a function" error when importing slugify from TypeScript #196 “"not a function" error when importing slugify from TypeScript”

Those cover callability/interop; this one is specifically about the declare module slugify syntax becoming a hard error under the newer compiler.

What would help

• Updating the declaration file to use the modern form (e.g. declare namespace or an ES module style) so it doesn’t trigger TS1540 under TS 6+ / @typescript/native-preview.
• Alternatively, a patch/minor release with updated .d.ts that’s compatible with Next.js 16’s default TypeScript toolchain.

In the meantime I can work around this by shadowing the types or patching slugify.d.ts locally, but having an official fix in the package would make life a lot easier for projects on the new TS compiler.

[Scalahansolo]

I ran into this yesterday as well. Was going to open a PR in this repo today and hope it gets merged. It's a one line change.

Here is the patch I have applied locally for now to move my repos forward.

diff --git a/slugify.d.ts b/slugify.d.ts
index 1234567..abcdefg 100644
--- a/slugify.d.ts
+++ b/slugify.d.ts
@@ -1,4 +1,4 @@
-declare module slugify {
+declare namespace slugify {
   type ExtendArgs = {
     [key: string]: any;
   }

[Trott]

If anyone knows if the slug module has the same problem or not, I'd appreciate a heads up. (I help maintain slugify here and there, but I'm the sole maintainer of slug.) I was unable to reproduce the issue on slugify with the minimal reproduction provided by OP here, but I assume that's because I was missing something simple about the setup that is implied and was going to get around to figuring that out yesterday, and then the day got away from me.

[fringe4life]

Hi, sorry if this issue is incomplete it's my first issue outside of my own account. Here is a link to my repo: firstroad. If that helps? Running bun type at the root - it's a Monorepo - triggers the typescript issue in the utils package where slugify is installed. This issue only happens for fairly recent versions of tsgo as it didn't use to occur before a recent update.

[fringe4life]

As for the slug package I am not sure. You would need to install a recent version of tsgo or maybe typescript 6 beta to test it out.

[fringe4life]

useful link