Why are my typescript interfaces being compiled to javascript?

Question

As I understand it, interfaces are only relevant in Typescript and the tsc compiler should be smart enough not to convert them to JS files in the final output. When I compile my code the interfaces are being compiled with it. Why is this?

My simplified project structure

src/
  index.ts
  lib/
    EventClient.ts
  interfaces/
    EventClientConfig.ts

What it compiles to

dist/
  index.js
  lib/
    EventClient.js
  interfaces/
    EventClientConfig.js

My tsconfig.json

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "outDir": "./dist",
    "lib": ["ES6", "DOM"],
    "target": "ES2018",
    "module": "CommonJS",
    "moduleResolution": "node",
    "esModuleInterop": true,
    "resolveJsonModule": true,
    "baseUrl": ".",
    "removeComments": true,
    "strict": true,
    "typeRoots": ["./node_modules/@types"],
    "rootDir": "./src",
    "types": [
      "node",
      "jest-fetch-mock",
      "express",
      "reflect-metadata"
    ]
  },
  "include": ["src"],
  "exclude": ["dist", "node_modules", "**/*spec.ts"]
}

An interface

export interface EventClientConfig {
  endpoint: string;
  authToken: string;
}

How I'm using the interface

import { EventClientConfig } from '../interfaces/EventClientConfig';

const config: EventClientConfig = {
  endpoint: '/api',
  authToken: '123456'
}

What the interface is compiled to

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });

In the compiled code EventClient.js makes no actual reference to the EventClientConfig.js so it's definitely not required. Why is Typescript compiling the interface files to JS?

Answer 1

This was solved thanks to @AluanHaddad who suggested using .d.ts declaration files. I have changed all my interface filenames so that they end with .d.ts and they are no longer included in the output.

Answer 2

The answer to your questions is here modules

In TypeScript, just as in ECMAScript 2015, any file containing a top-level import or export is considered a module. Conversely, a file without any top-level import or export declarations is treated as a script whose contents are available in the global scope (and therefore to modules as well).