What method does TypeScript use to locate a .d.ts file if it is not specified in the configuration?

Recently, I encountered an issue with my TypeScript file when importing an mdx file.

import mdx from "./Layout.mdx";

After some research, I found a helpful resource that suggested a solution:

If you’re getting errors from TypeScript related to imports with an *.mdx extension, create an mdx.d.ts file in your types directory and include it inside your tsconfig.json.

// types/mdx.d.ts
declare module '*.mdx' {
  let MDXComponent: (props: any) => JSX.Element
  export default MDXComponent
}

Implementing this solution, I updated my folder structure as follows:

Root
-types
--mdx.d.ts
-src
--My tsx files

Surprisingly, the compiler stopped complaining about the import. However, I was puzzled by how it automatically recognized the d.ts file in the types folder, as I didn't reference it in my tsconfig.json:

{
  "compilerOptions": {
    "rootDir": "src",
    "declaration": true,
    "declarationDir": "build",
    "module": "esnext",
    "target": "es5",
    "lib": ["es6", "dom", "es2016", "es2017"],
    "sourceMap": true,
    "jsx": "react",
    "moduleResolution": "node",
    "allowSyntheticDefaultImports": true,
    "esModuleInterop": true,
    "noImplicitAny": true
  },
  "include": ["src/**/*"],
  "exclude": [
    "node_modules",
    "build",
    "storybook-static",
    "src/**/*.stories.tsx",
    "src/**/*.test.tsx"
  ]
}

It's worth noting that the issue was highlighted in Visual Code IDE, but the error disappeared once I placed the d.ts file in the types folder.

UPDATE: For further reference, here is the link to the package.json

Answer №1

When it comes to Typescript, the types directory is not automatically included, leading to the issue of not finding the d.ts file. The compilation process includes specific files, as detailed in an updated manner here: https://www.typescriptlang.org/tsconfig#Project_Files_0

It's important to note that the included files act as the starting point. Any imports from these files will also be included in the compilation, even if they are in an "excluded directory."

Consequently, the exclusions listed in your tsconfig.json [

"node_modules", "build", "storybook-static"
] do not have an impact since they are already "not included" by "include": ["src/**/*"].

After examining your project, running npm ci, and compiling with npx tsc, I can confirm that Typescript does not include the types/mdx.d.ts file in the compilation process. However, the compilation still works smoothly even without it.

If you encountered discrepancies, it could be related to IDE settings or a compilation cache issue. In such cases, consider the following questions:

  • Have you verified that your IDE is using the Typescript version from node modules and not a globally installed version?
  • Have you tried a fresh project checkout followed by Typescript compilation with npx tsc, removing types/mdx.d.ts, and compiling again with npx tsc?

Answer №2

Once I made that adjustment, my compiler stopped throwing errors related to the import statement.

Although your IDE may not be displaying any complaints (IDEs often automatically include open files in the compilation context for convenience), the compiler will still flag any issues. To run the compiler, you can use the following command:

tsc -p ./tsconfig.json

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