Typescript’s declaration merging not working as expected using ts-node

For a project using the express-session package, I’m trying to mutate the session object by simply adding a user key.

req.session.user = 123;

Coming from this question’s accepted answer, I understand I could use declaration merging to extend the SessionData interface, using my own interface.

Looking at various open-source projects, such as the HospitalRun components repository I notice them having the types directory in their tsconfig.json file under the include section like this.

  "include": [
    "src",
    "types"
  ]

My whole tsconfig.json looks like this, which lives in the root of the project.

{
    "include": [
        "types",
        "src",
    ],
    "exclude": [
        "node_modules"
    ],
    "compilerOptions": {
        "lib": [
            "esnext",
            "esnext.asynciterable"
        ],
        "baseUrl": ".",
        "skipLibCheck": true,
        "module": "commonjs",
        "esModuleInterop": true,
        "target": "es6",
        "moduleResolution": "node",
        "outDir": "build",
        "experimentalDecorators": true,
        "emitDecoratorMetadata": true,
        "allowSyntheticDefaultImports": true,
        "strict": true,
        "strictPropertyInitialization": false,
    },
}

I tried doing the same, together a file called express-session.d.ts in the root of this folder (~/types/), having the following contents:

import session from 'express-session';

declare module 'express-session' {
    interface SessionData {
        user: any;
    }
} 

However, the error I keep receiving is this.

 Property 'user' does not exist on type 'Session & Partial<SessionData>'

When I do however add this piece of code above the code I use for mutating my session object, I no longer have the problem. This doesn’t seem like the right approach though.

Also, when I use tsc src/index.ts --build instead of ts-node src/index.ts it also works.

What am I doing wrong here? How can this be fixed? I also tried using the typeRoots, using the same folder.

Answer

LATEST UPDATE (08-MAY-2021)

When running the typescript program by using ts-node, even typeRoots are specified in tsconfig.json, it cannot recognise the custom .d.ts and prompt Property 'x does not exist on type y` error.

According to https://github.com/TypeStrong/ts-node/issues/1132#issuecomment-716642560

One of the contributors of ts-node suggested multiple ways to solve it.

Here is one of it: Specifying file: true flag in tsconfig.json to inform ts-node to load files, include and exclude options from tsconfig.json on startup

{
  "ts-node": {
    "files": true
  },
  "exclude": [...],
  "compilerOptions": {
   ...
}

---

OLD: (07-MAY-2021)

There is no need to use include in tsconfig.json and the paths are not correct. The compiler can search the ts file in the directory and sub-directories

Try to remove it. and restart TS server.

If you are using VSCode, try Cmd + Shift + P or Ctrl + Shift + P and search Restart TS server and see if the user type error still exist

{
    "exclude": [
        "node_modules"
    ],
    "compilerOptions": {
        "lib": [
            "esnext",
            "esnext.asynciterable"
        ],
        "baseUrl": ".",
        "skipLibCheck": true,
        "module": "commonjs",
        "esModuleInterop": true,
        "target": "es6",
        "moduleResolution": "node",
        "outDir": "build",
        "experimentalDecorators": true,
        "emitDecoratorMetadata": true,
        "allowSyntheticDefaultImports": true,
        "strict": true,
        "strictPropertyInitialization": false,
    },
}