TypeScript: cannot write file .d.ts because it would overwrite input file

October 16, 2021

There’s countless issues about this error and I thought it would be useful to write a clear explanation of what’s going on and a summary of the possible solutions.

This happens for two reasons:

  1. You’re explicitly including those .d.ts files e.g. as part of your include array in tsconfig.json or as part of the tsc arguments, and you’re asking TypeScript to output type declarations in the same place. Then the error is pretty obvious and the fix should be too (don’t output the generated declarations in the same place as types you’re importing, for example using outDir, or don’t import those generated declarations in the first place).
  2. You’re not importing those .d.ts files, or you’re even explicitly ignoring them e.g. with the exclude array in tsconfig.json, yet TypeScript keeps using them as input and complaining that it can’t overwrite them when generating type declarations.

Here we’ll go in more details about the second reason.

The problem

The main thing that you need to know is that if you’re importing a .js file and there’s a matching .d.ts next to it, TypeScript will always import it, even if you didn’t explicitly include those .d.ts files as input, and even if you explicitly put them in the exclude array. There’s no way around this.

Output .d.ts declarations to a separate directory

One solution is to put the generated type declarations in a separate directory instead of next to the .js files. You can do that by configuring a outDir, as explained in the documentation about creating .d.ts files from .js files:

  "compilerOptions": {
    // Tells TypeScript to read `.js` files, as normally they are
    // ignored as source files.
    "allowJs": true,
    // Generate `d.ts` files.
    "declaration": true,
    // This compiler run should only output `d.ts` files.
    "emitDeclarationOnly": true,
    // Types should go into this directory. Removing this would place
    // the `.d.ts` files next to the `.js` files.
    "outDir": "dist"

As they nicely indicate, if you don’t specify outDir, the .d.ts will be put next to the .js files (which literally means they’ll be automatically considered as inputs on the next build and it will crash), and it’s probably the way you’re using this right now.

Then you can tell TypeScript where to find the package types in your package.json:

  "types": "dist"

But this only works for the main export (import 'my-lib') and will break if you attempt to import nested files import 'my-lib/some-file'.

If you want to support this use case, you have to ship the .d.ts files next to the .js files.

So here’s a few alternative solutions and their tradeoffs.

Copy the .js files next to the .d.ts declarations

Since we can’t generate the .d.ts next to the source .js files (well we can, but just once), we can instead generate the .d.ts files to a dist directory and copy the .js files next to them.

There’s two ways you can do that, the first one I tried is to remove emitDeclarationOnly so that let TypeScript compiles the source .js files to the outDir, and the other one is to manually copy them.

In both cases there’s a number of caveats with that about how you import nested files, and I’ll go through the possible workarounds.

Compile your JS files to JS (lol)

The reason you have this error in the first place is likely because you’re writing actual JavaScript and generating types from JSDoc.

One of the numerous benefits of doing that is that you don’t need to compile your code. Your src is your dist and that’s the beauty of it. You run what you write, no compilation, no source maps, and no configuration of every single tool and service you use to deal with this extra complexity.

You can throw away all of those benefits by letting TypeScript compile your .js files to the outDir, by removing emitDeclarationOnly from the tsc command or tsconfig.json, so that they’re put along the generated .d.ts files.

But at that point you might as well write TypeScript in the first place.

Manually copy your JS files to the outDir

A better way if you want to ship your .js files unaltered is to copy them yourself next to the .d.ts declarations.

tsc *.js --allowJs --declaration --emitDeclarationOnly --outDir dist && cp *.js dist

Then you can import 'my-lib/dist/some-file and types will work properly. If you want to allow deep imports though, we need to dig a bit further.

Getting it to work with deep/nested imports

If you want to allow import 'my-lib/some-file' and don’t like the idea of documenting import 'my-lib/dist/some-file', you have again a few options.

Compile to the project root

Make sure your source files are in a subfolder, e.g. src, then compile to the project root directory.

tsc src/*.js --allowJs --declaration --emitDeclarationOnly --outDir . && cp src/*.js .

Publish from your dist directory

The previous solution might get a bit messy though so alternatively you can use the earlier command with --outDir dist, but put your package.json in the dist directory as well, and run npm publish dist (or cd dist && npm publish).

Whether you want your package.json to live in the dist directory (and commit it there), or run cp package.json dist as part of your build command is up to you.

Write an exports map

If you’re not happy with the previous solutions, you can write an exports map in your package.json so that import 'my-lib/some-file translates to my-lib/dist/some-file.

  "exports": {
   "./some-file": "./dist/some-file",
   "./some/other-file": "./dist/some/other-file"

That being said only the paths defined here will be allowed to be imported, you won’t be able to import arbitrary files anymore, which might not be a bad thing, but maybe you like the simplicity of everything being importable by default.

Quick and dirty hack that actually works

To get the best of both worlds by generating .d.ts files next to your source .js files without adding extra configuration and still allowing deep imports, you need to explicitly remove the generated files before running the compiler.

Simple, easy and dirty:

rm -f *.d.ts && tsc *.js --allowJs --declaration --emitDeclarationOnly

Here I use rm -f so that it doesn’t fail if the declaration files are not generated yet. Feel free to tweak the pattern, for example if you have subfolders you want to include.

I’m not a big fan of this solution, but it’s still my favorite of all the ones I described in this post. It seems that TypeScript wasn’t built for simplicity, let alone for working with source .js files, and deep imports don’t seem to be part of the happy path either. If you found a better way, please let me know!

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