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

Question

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 | });

Hi Jonathan, is it possible that you pass a sample of apiProducts in order to reproduce this error? also could you provide the exact error you get in the console? — Mario Perez, Jun 23 '20 at 22:23
Weird thing i Noticed about your constructor Object.assign(this, obj: Object) <-- would do everything you perfomed manually :D — kristjan reinhold, Sep 08 '21 at 08:27

score 210 · Answer 1 · answered Jun 23 '20 at 22:20

210

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']);

answered Jun 23 '20 at 22:20

Mario Perez

2,777
1
12
21

31

using `toEqual` will also works, in conclusion, do not use `toBe` to compare arrays. – Muhaimin Taib Jun 03 '22 at 15:30
Back when I posted I think the toEqueal method didn’t cut it, I’ll have a look at it – Mario Perez Jun 04 '22 at 16:33
2

you also need to do .sort() at each array to make sure you compare them in the same sequence – Va5ili5 Apr 11 '23 at 15:39

score 49 · Answer 2 · answered Nov 05 '20 at 08:05

49

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));

answered Nov 05 '20 at 08:05

Juangui Jordán

6,091
2
35
31

7

`expect(newDeal).toMatchObject(expected)` will do. No need to convert to string. – ACV Dec 06 '21 at 18:04
3

@AVC Are you sure that's correct? Jest says this about `expect(newDeal).toMatchObject(expected)`: `It will match received objects with properties that are not in the expected object.`. It did not work for me when I tried it. I had to do `JSON.stringify()` on both sides of the equation in order for the `toEqual()` comparison to succeed. – Jose Quijada Dec 14 '21 at 22:22
1

`toMatchObeject()` works nicely. Thanks. – PowerAktar Sep 27 '22 at 10:31
1

Somehow toMatchObeject() is not working for me. expect(JSON.stringify(newDeal)).toMatchObject(JSON.stringify(expected)); is working fine and makes the test passed. – Gavara.Suneel Dec 08 '22 at 07:21

score 38 · Answer 3 · answered Jul 27 '19 at 08:21

38

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.

answered Jul 27 '19 at 08:21

Maksim Nesterenko

5,661
11
57
91

1

My problem was that we'd put a static property on our array, which is similar to this – gonzofish Apr 29 '21 at 16:41
1

Thanks for this answer, ran into this exact scenario! – Azarro Jul 09 '22 at 19:31

score 14 · Answer 4 · answered Nov 17 '21 at 11:13

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,
                );

score 13 · Answer 5 · answered Jun 07 '21 at 06:28

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({})

score 6 · Answer 6 · answered Jul 22 '22 at 14:50

6

I had a similar issue while comparing two MongoDb ObjectIds. To overcome the problem, I used

expect(`${thisObject._id}`).toEqual(`${thatObject._id}`);

answered Jul 22 '22 at 14:50

devdamnit

69
1
1

Prashant Rawat · Answer 7 · 2022-07-18T16:03:07.453

I had this same issue with jest. In my case I was comparing the array of objects (basically a model class). And in that class I had defined a function as an arrow function.

The problem is, while comparing it checks for the arrow functions also. And as arrow functions create different instances for all the objects in contrast to normal function which have only one instance class-wide, the arrow function comparison results false.

So a simple solution would be to convert your arrow functions to normal functions in classes.

Use:

class ClassName {
  yourFunc(){
    // your code here
  }
};

Instead of:

class ClassName {
  yourFunc = () => {
    // your code here
  };
};

So once converted to normal function you can simply use toEqual() for comparison.

If you can't convert to normal function you can use JSON.stringify() to convert them first to strings and then use toEqual() or toBe().

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

deal.test.js

Deal.js

7 Answers7

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