Cannot Load Custom Definition File in Repository

TypeScript Version: 2.8.1

Search Terms: Definition File, Ambient type Detail I am importing a JavaScript library that doesn’t have any definition file. I want to create a custom definition file that I place in my repository. I want TypeScript to read this definition file.

I have tried many combination of tsconfig.json (typeRoot, include, path, baseUrl, etc.) Without being successful. I also moved the definition folder under src, and renamed the definition file to index.d.ts (inside a folder with the name of the library) without any success.

Code Small complete repro here: https://github.com/MrDesjardins/importdefinitionfiles

// main.ts:
import MessageFormat from "messageformat"; // Doesn't have type
// messageformat.d.ts:
declare module messageformat {
    export type Msg = (params: {}) => string;
    export interface MessageFormat {
        new(message: string): any;
        compile: (messageSource: string) => Msg;
    }
}

Expected behavior: TypeScript to find the definition file and to use it without having a compilation error.

Actual behavior: Error message

Could not find a declaration file for module 'messageformat'. '/Users/pdesjardins/code/perso/importdefinitionfiles/node_modules/messageformat/lib/messageformat.js' implicitly has an 'any' type.
  Try `npm install @types/messageformat` if it exists or add a new declaration (.d.ts) file containing `declare module 'messageformat';`

1 import MessageFormat from "messageformat";

Playground Link: https://github.com/MrDesjardins/importdefinitionfiles

Related Issues:

[RyanCavanaugh]

You need quotes around the name:

declare module 'messageformat' {

The error message tells you this:

> add a new declaration (.d.ts) file containing `declare module 'messageformat';`

In the future please use StackOverflow for questions.

[RyanCavanaugh]

In fact we can check its declaration to see if it’s written as declare module foo.

Cannot find module "foo"
  An object in the global scope named 'foo' exists and can be accessed without importing it.

or (if we see declare module)

Cannot find module "foo"
  A declaration for the *global* 'foo' object exists, but was written as "declare module foo". Did you mean to write 'declare module "foo"' (with quotes)?