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

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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 | });
jestjs unit-testing javascript
7 Answers

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:

screenshot of jest results

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']);
user_64655910

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.

user_26701210

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));
user_6271610

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,
                );
user_55381400

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({})
user_43858890

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

expect(`${thisObject._id}`).toEqual(`${thatObject._id}`);
user_196023250

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().

user_117762850
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