esmload

This module can be use to mock the results of a native ES6 import in Node.js.

Install

$ npm i -D esmload

Usage

• Node.js process needs to be started with the following flags:

$ node --experimental-modules --loader esmload <index.mjs>

• After that, all imported modules will have an export named 'TEST_MOCK':

import { main, TEST_MOCK } from './lib1.mjs';

TEST_MOCK is a map:

• each entry has the name of an export from the module.
• each value is a Proxy handler that can be used to manipulate the linked export.

Example

// lib1.mjs
export function main() {
    return 'hello World!';
}

// index.mjs
import { main, TEST_MOCK } from './lib1.mjs';
 
console.log(main()); // prints 'hello World!'
const mainProxyHandler = TEST_MOCK.get('main');
mainProxyHandler.apply = function(target, self, args) {
    return Reflect.apply(target, self, args) +  '!!';
}
console.log(main()); // prints 'hello World!!!'
mainProxyHandler.apply = undefined;
console.log(main()); // prints 'hello World!'

See tests for other examples.

Limitations

Right now, you can't mock something that is not an object.

Status

This is a PoC I just wrote in a couple hours. Next steps may include:

• provide a higher level interface hide the proxies to most users
• propose a dependency injection system with the same bases
• inline that in a test framework somehow