How To Mock a Dependency Class in Typescript Node Using Chai-Spies - node.js

Question

Given something like my-module.ts that will be called by an external consumer that of course cannot do dependency injection to this module's constructor:
import Dependency from 'dependency-class';
import {myDataFormat} from '../types/my-module-types';
export default class MyModule {
private dependency: Dependency;
constructor() {
this.dependency = new Dependency();
}
getMyData(): myDataFormat {
const dependencyData = this.dependency.getDependencyData();
// parse/process data
// return modified data
}
}
How can we test that Dependency.getDependencyData() was called by MyModule.getMyData()... using only mocha, chai, and chai-spies?

Answer 1

my-module.test.ts:
import Dependency from 'dependency';
import MyModule from '../src/modules/my-module'
import 'mocha';
import chai = require('chai');
chai.should(); //enable chai should()
import spies = require('chai-spies');
chai.use(spies); //extend chai with spies plugin
describe('Tests.MyModule', () => {
let instance: MyModule | undefined;
before(() => {
// Create spies/stubs/mocks as needed
chai.spy.on(Dependency.prototype, ['getDependencyData'], () => {
// this is replacement function body if we want to test specific return data
});
// Create MyModule instance
instance = new MyModule();
});
after(() => {
// Reset MyModule instance
instance = undefined;
// Remove spies/stubs/mocks that were created
chai.spy.restore();
});
describe('getMyData()', () => {
it('should call Dependency.getDependencyData()', () => {
// Arrange
// - Set up spies/stubs/mocks (done above)
// - Create a MyModule instance (done above)
// Act
instance.getMyData();
// Assert
Dependency.prototype.getDependencyData.should.have.been.called();
});
});
});
Versions in Node package.json:
"#types/chai": "^4.3.0",
"#types/chai-spies": "^1.0.3",
"#types/mocha": "^9.1.0",
"chai": "^4.3.6",
"chai-spies": "^1.0.0",
"mocha": "^9.2.1",