why declaration can not be used together with isolatedModules in typescript?

Question

declaration is used to generate type file, and isolatedModules mean all file should be a separate module. Why do these two options use together?

error TS5053: Option 'declaration' cannot be specified with option 'isolatedModules'.

Answer 1

Answer

In the issue that Max Heiber referenced in the comments, Wesley Wigham from the TypeScript team posted the following answer (emphasis added):

The reason is for the same reason you can't use const enums in isolated modules: type information. Since isolated modules compiles each file individually without the types of the files it depends on, any inferred type we would write to a declaration file would potentially be incorrect, as their calculation would be missing information from the rest of the compilation. There is a limited subset of declaration emit where no types are inferred in the output which could be supported, however.

In other words, isolatedModules does not provide enough type information for the creation of complete and accurate *.d.ts declaration files.

Suggested Workaround

The issue comments also have a suggested workaround, in which we have one tsconfig for compiling with isolated modules, and a second tsconfig for creating declaration files.

tsconfig.json

{
    "compilerOptions": {
        "incremental": true,
        "isolatedModules": true
    }
}

tsconfig-for-declarations.json

{
    "extends": "./tsconfig",
    "compilerOptions": {
        "emitDeclarationOnly": true,
        "isolatedModules": false,
        "declaration": true
    }
}