Jest.js error: "Received: serializes to the same string"

I've having a strange problem with this test:

deal.test.js

import Deal from "../src/models/Deal";
import apiProducts from "../__mocks__/api/products";

describe("Deal", () => {
  describe("Deal.fromApi", () => {
    it("takes an api product and returns a Deal", () => {
      const apiDeal = apiProducts[0];
      const newDeal = Deal.fromApi(apiDeal);
      const expected = expectedDeal();
      expect(newDeal).toEqual(expected);
    });
  });
});

Deal.js

export default class Deal {
  // no constructor since we only ever create a deal from Deal.fromApi

  static fromApi(obj: Object): Deal {
    const deal = new Deal();
    deal.id = obj.id;
    deal.name = obj.name;
    deal.slug = obj.slug;
    deal.permalink = obj.permalink;
    deal.dateCreated = obj.date_created;
    deal.dateModified = obj.date_modified;
    deal.status = obj.status;
    deal.featured = obj.featured;
    deal.catalogVisibility = obj.catalog_visibility;
    deal.descriptionHTML = obj.description;
    deal.shortDescriptionHTML = obj.short_description;
    deal.price = Number(obj.price);
    deal.regularPrice = Number(obj.regular_price);
    deal.salePrice = Number(obj.sale_price);
    deal.dateOnSaleFrom = obj.date_on_sale_from;
    deal.dateOnSaleTo = obj.date_on_sale_to;
    deal.onSale = obj.on_sale;
    deal.purchasable = obj.purchasable;
    deal.relatedIds = obj.related_ids;
    deal.upsellIds = obj.upsell_ids;
    deal.crossSellIds = obj.cross_sell_ids;
    deal.categories = obj.categories;
    deal.tags = obj.tags;
    deal.images = obj.images;
    return deal;
  }

 descriptionWithTextSize(size: number): string {
    return this.descriptionWithStyle(`font-size:${size}`);
  }

  descriptionWithStyle(style: string): string {
    return `<div style="${style}">${this.description}</div>`;
  }

  distanceFromLocation = (
    location: Location,
    unit: unitOfDistance = "mi"
  ): number => {
    return distanceBetween(this.location, location);
  };

  distanceFrom = (otherDeal: Deal, unit: unitOfDistance = "mi"): number => {
    return distanceBetween(this.location, otherDeal.location);
  };

  static toApi(deal: Deal): Object {
    return { ...deal };
  }
}

The test fails with this error:

  ● Deal › Deal.fromApi › takes an api product and returns a Deal

    expect(received).toEqual(expected) // deep equality

    Expected: {"catalogVisibility": "visible", "categories": [{"id": 15, "name": "New York", "slug": "new-york"}], "crossSellIds": [34, 31], "dateCreated": "2019-05-18T17:36:14", "dateModified": "2019-05-18T17:39:02", "dateOnSaleFrom": null, "dateOnSaleTo": null, "descriptionHTML": "<p>Pete's Tavern<br />
    129 E 18th St<br />
    New York, NY 10003</p>
    <p>Weekdays from 4 p.m. to 7 p.m.<br />
    $5 wines and beers</p>
    ", "distanceFromLocation": [Function anonymous], "featured": false, "id": 566, "images": [{"alt": "", "date_created": "2019-05-18T17:38:52", "date_created_gmt": "2019-05-18T17:38:52", "date_modified": "2019-05-18T17:38:52", "date_modified_gmt": "2019-05-18T17:38:52", "id": 567, "name": "wine and beers2", "src": "https://tragodeals.com/wp-content/uploads/2019/05/wine-and-beers2.jpg"}], "name": "Wines and beers", "onSale": true, "permalink": "https://tragodeals.com/product/wines-and-beers/", "price": 5, "purchasable": true, "regularPrice": 11, "relatedIds": [552, 564, 390, 37, 543], "salePrice": 5, "shortDescriptionHTML": "<p>$5 wines and beers</p>
    ", "slug": "wines-and-beers", "status": "publish", "tags": [{"id": 58, "name": "beers", "slug": "beers"}, {"id": 54, "name": "Cocktails", "slug": "cocktails"}, {"id": 45, "name": "drink", "slug": "drink"}, {"id": 57, "name": "wine", "slug": "wine"}], "upsellIds": [53]}
    Received: serializes to the same string

    > 15 |       expect(newDeal).toEqual(expected);
         |                       ^
      16 |     });
      17 |   });
      18 | });

      at Object.toEqual (__tests__/deal.test.js:15:23)

I inserted this loop to investigate:

for (let key in expected) {
  expect(expected[key]).toEqual(newDeal[key]);
}

And I see that the problem is with functions. So I changed the whole test to this:

      for (let key in expected) {
        if (typeof expected[key] === "function") continue;
        expect(expected[key]).toEqual(newDeal[key]);
      }
     // expect(newDeal).toEqual(expected);

And it passes, and also fails when it should. (if you read the old version of this question where I was getting passing tests that I didn't understand, it was because I was returning from the loop when I should have been continueing).

But I'd like to be able to do it with the standard assertion expect(newDeal).toEqual(expected). It looks like there's something I'm not understanding about checking for class object (Deal) equality with functions.

PS. You might suggest using toMatchObject. But, sadly:

  ● Deal › Deal.fromApi › takes an api product and returns a Deal

    expect(received).toMatchObject(expected)

    - Expected
    + Received

    @@ -1,6 +1,6 @@
    - Deal {
    + Object {
        "address": "129 E 18th St New York, NY 10003",
        "catalogVisibility": "visible",
        "categories": Array [
          Object {
            "id": 15,

      13 |         expect(expected[key]).toEqual(newDeal[key]);
      14 |       }
    > 15 |       expect(newDeal).toMatchObject(expected);
         |                       ^
      16 |     });
      17 |   });
      18 | });

Similarly to other colleagues I had this issue with an Array comparison, I was basically testing a function that got the largest string in an array, additionally it should return an array if more than 1 of those strings matched the largest length possible.

When I started testing I got the following message:

So I replaced the toBe method

expect(function(array1)).toBe('one result')

with toStrictEqual to make a deep equality comparison

expect(function(array2)).toStrictEqual(['more than one', 'more than one']);

I had a similar case where the object had a base64 encoded string, I managed the test to compare the serialization of the object using JSON.stringify:

expect(JSON.stringify(newDeal)).toMatchObject(JSON.stringify(expected));

Just had this problem when tried to compare arrays where in one array there was an element with -1 index set (imagine any other key to be set except numbers from 0 to N). So you may have this error in the following scenario:

const arr = [1, 2]
arr[-1] = 'foo'
expect(arr).toEqual([1, 2])

They both serialized to the same string, but they are not equal.

In my situation, I was deep equal checking a proxied object vs a regular object.

const proxyObject = new Proxy({}, {
    get: function(target, prop) {
        return Object.hasOwnProperty.call(target, prop) ? target[prop] : '';
    }
})
expect(proxyObject).toEqual({})
// Expected: {}
// Received: serializes to the same string

It should be:

expect(proxyObject).toMatchObject({})

I had this problem when i tried to compare arrays where one array was coming back from the Graphql's resolver and the other one was from my test's input.
ALL the fields were the same except the entries inside the array coming from Graphql did not have any __proto__ while the ones from my test input had __proto__: Object and that cause the toStrictEqual to fail, because it checks the types besides the structure. So, in my case the type caused to fail. Changing it to toEqual solved the problem.

				expect(res.data.myMutation.values).toEqual(
					testInput.values,
				);