Mock imported class from module using sinon

README.md

Context

Sinon design is mostly focused on mocking/stubbing function instead of class definitions. Its easy to mock class functions (via mocking/stubbing the prototype) but having a side effect on constructor makes things complicated.

Solution

One work around to this is using the sinon.createStubInstance(constructor) method and then add a fake call to the class.

collection.js

const rectangle = require("./rectangle.js");

class RectangleCollection {
  constructor(rectangles) {
    this.rectangles = [];
    for (const rec in rectangles) {
      this.rectangles.push(new rectangle.Rectangle(rec[0], rec[1]));
    }
  }

  getArea() {
    return this.rectangles.reduce((rec) => rec.getArea(), 0);
  }
}

module.exports = {
  RectangleCollection,
};

rectangle.js

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }

  getArea() {
    return this.width * this.height;
  }
}

module.exports = {
  Rectangle,
};

test.js

const assert = require("assert");
const sinon = require("sinon");
const collection = require("./collection.js");
const rectangle = require("./rectangle.js");

describe("Area test", function () {
  it("should calculate the area of a collection", function () {
    const stubbedRectangle = sinon.createStubInstance(rectangle.Rectangle, {
      getArea: sinon.stub().returns(1),
    });
    sinon.stub(rectangle, "Rectangle").callsFake((...args) => {
      console.log("Mock called");
      return stubbedRectangle;
    });
    const c = new collection.RectangleCollection([(2, 5), (3, 4), (1, 9)]);
    assert(c.getArea, 3);
  });
});