Jest mock of a singleton function

Question

I have a singleton class created in a module consumed within a class of an app, I am attempting to change the mock implementations return value of the singletons contained function, in the example below I've named it isDecision.

I have tried spying on the singleton

jest.spyOn(MySingleton, "getInstance").mockImplementation({
  isDecision: jest.fn(() => false),
});

calling and mocking inline

MySingleton.getInstance().isDecision.mockReturnValue(false);

I am really struggling to understand what the process is for changing the implementation of a singleton inner functions is.

EXAMPLE STRUCTURE

export default class MySingleton {
    
  private static _instance: MySingleton;

  static getInstance(): MySingleton {
    if (!MySingleton._instance) {
      MySingleton._instance = new MySingleton();
    }

    return MySingleton._instance;
  }

  isDecision(): boolean {
    return true;
  }
}

class usage


import { MySingleton } from "@my-singleton";

export class Consumer {
  
  private _singleton: MySingleton;

  constructor() {
   this._singleton = MySingleton.getInstance();
  }

  utilityFunction(): boolean {
    return this._singleton.isDecision();
  }
}

test cases within Consumer for isDecision

jest.mock("@my-singleton", () => ({
  __esModule: true,
  MySingleton: {
    getInstance: jest.fn().mockImplementation(() => ({
      isDecision: jest.fn(() => true),
    })),
  },
}));

describe("Consumer", () => {
  let consumer;

  beforeEach(() => {
    consumer = new Consumer();
  });
  
  describe("test decision is true case", () => {
    
    test("should return true", () => {
      const decision = consumer.utilityFunction(); 
      expect(decision).toBe(true);
    });
  });

  describe("test decision is false case", () => {
    
    beforeEach(() => {
      // how to change IsDecsion of singleton to return false?
    });

    test("should return true", () => {
      const decision = consumer.utilityFunction(); 
      expect(decision).toBe(false);
    });
  });
});

Deviland · Accepted Answer · 2024-08-23 16:16:36Z

0

I found a workaround more than an answer

import { MySingleton } from "@my-singleton";

export class Consumer {
  utilityFunction(): boolean {
    return MySingleton.getInstance().isDecision();
  }
}

If I remove the Singleton as a class property and use it directly as above jest can now mock the isDecision function at will. Hope this pain helps someone else

answered Aug 23, 2024 at 16:16

Deviland

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