Typescript: after the transpilation, wrong retun type

Why my index.ts

export const isNullOrUndefined = (value: any): value is null | undefined => {
  return value === null || value === undefined;
};

is transpiled into index.d.ts (only value is null)

export declare const isNullOrUndefined: (value: any) => value is null;

instead of (value is null | undefined)

export declare const isNullOrUndefined: (value: any) => value is null | undefined;

this is the tsconfig file on typescript 4.9.5

{
  "compilerOptions": {
    "incremental": false,
    "target": "es2019",
    "outDir": "build",
    "rootDir": "src",
    "moduleResolution": "node",
    "module": "commonjs",
    "declaration": true,
    "inlineSourceMap": false,
    "esModuleInterop": true,
    "resolveJsonModule": true,
    "removeComments": false,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "traceResolution": false,
    "listEmittedFiles": false,
    "listFiles": false,
    "pretty": true,
    "lib": ["es2019", "dom"],
    "types": ["node"],
    "typeRoots": ["node_modules/@types", "src/types"]
  },
  "include": ["src/**/*.ts"],
  "exclude": ["node_modules/**"],
  "compileOnSave": false
}

>Solution :

It’s because you don’t have strictNullChecks enabled. From the option’s documentation:

When strictNullChecks is false, null and undefined are effectively ignored by the language. This can lead to unexpected errors at runtime.

When strictNullChecks is true, null and undefined have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected.

Add at least:

"strictNullChecks": true,

to your tsconfig.json (although I’d recommend all of the strict flags, via "strict": true).