Why does this custom type definition not properly export a function?

Question

I have an npm dependency (electron-is-accelerator) in my Typescript project that comes with no types, neither as part of the package, nor as part of DefinitelyTyped. The dependency consists of a module with just one function accepting a string and returning a boolean. I wrote a type definition in src/@types/electron-is-accelerator as described in this article:

declare module 'electron-is-accelerator' {
    export function isAccelerator(s: string): boolean;
}

I tried to execute this function like this:

import { isAccelerator } from 'electron-is-accelerator';

console.log(isAccelerator("Alt+Space")); 

But I get the following runtime error:

TypeError: electron_is_accelerator_1.isAccelerator is not a function

How can I get this to run?

My tsconfig.json:

{
    "compilerOptions": {
        "outDir": "./built",
        "module": "commonjs",
        "target": "es6",
        "noImplicitAny": true,
        "sourceMap": false
    },
    "include": [
        "src/**/*"
    ]
}

I uploaded a minimal demonstration example here for anyone who wants to play with the problem.

Answer 1

That package uses CommonJS module.exports syntax to export the function:

module.exports = function (str) { ... };

module.exports is different from ES modules default export syntax and your named export declaration in the example above, so TS has export =, import = require() to express these imports/exports. electron-is-accelerator can be typed with following declaration:

declare module "electron-is-accelerator" {
  function isAccelerator(s: string): boolean;

  export = isAccelerator;
}

The import looks like this:

import isAccelerator = require("electron-is-accelerator")

isAccelerator("whatever")

If esModuleInterop is enabled, you can write it as ES default import:

import isAccelerator from "electron-is-accelerator"