"Ambient context" in typescript terms means "anything that does not have an implementation", that does not emit javascript codes. It only exists in the type system at compile time, and is erased at run-time. "declare", "type", "interface" etc., are all ambient context.

A ".d.ts" contains only ambient context. Rule is:

• Initializers are not allowed in ambient contexts - may declare a type, but cannot initialize with value
• Top-level declarations in .d.ts files must start with either a 'declare' or 'export' modifier
• No import, but allows import() of types

declare const something:string; // ok

declare const somethingElse:string = "with value"; // wrong

import {User} from "./user" // wrong

declare namespace Express {

  interface Request {

    user: import("./user").User;  // ok

  }

}

To declare type for javascript (which has no type). It tells compiler to trust, at run time, there's a declared something as some type.

What works and not works (to place declaration file and config compiler option)

Works

• OK - an included ".ts", i.e. in "include" (https://www.typescriptlang.org/tsconfig#include)
• OK - an included ".d.ts", i.e. in "include" (https://www.typescriptlang.org/tsconfig#include)
• OK - if
  • put "anyFolder" under a path
  • put "index.d.ts" in "anyFolder" (index.ts does not work)
    • put declaration in "index.d.ts", or
    • put declaration in another ".d.ts", and reference it using `/// <reference path="./path/to/another.d.ts" />` in index.d.ts
  • put the path in "typeRoots"
  • no "types" compiler option, or put "anyFolder" in "types" (https://www.typescriptlang.org/tsconfig#types)

Not works:

• NOT WORK - place in a not included ".ts" file and place path of that file in "types" configutation (https://www.typescriptlang.org/tsconfig#types)
• NOT WORK - place in a not included, "something.ts" or "something.d.ts" file under "./somePath", and put "./somePath" in "typeRoots" (https://www.typescriptlang.org/tsconfig#typeRoots) without "types"
• NOT WORK - place in a not included, "something.ts" or "something.d.ts" file under "./somePath", and put "./somePath/something" in "typeRoots" (https://www.typescriptlang.org/tsconfig#typeRoots) without "types"
• NOT WORK - place in a not included, "index.ts" or "index.d.ts" file under "./somePath", and put "./somePath" in "typeRoots" (https://www.typescriptlang.org/tsconfig#typeRoots) without "types"
• error TS2688: Cannot find type definition file for 'anyFolder' - if put "anyFolder" under a path, the path in "typeRoots", and there is no "index.d.ts" inside it