Encountering test failures can be a developer’s daily bread, but some error messages are particularly puzzling. One such cryptic message in the Jest testing framework is “Received: serializes to the same string.” This error often leaves developers scratching their heads, as it suggests that two seemingly different values are, in Jest’s eyes, identical when converted to a string. It’s a common stumbling block, especially when dealing with complex objects, arrays, or snapshot tests. Understanding the root cause of this error is crucial not just for fixing the immediate problem, but for writing more robust and predictable tests in the long run. This deep dive will unravel the intricacies behind this message, offering clear explanations and actionable strategies to resolve it, ensuring your Jest tests accurately reflect your application’s behavior.

Understanding the “Received: serializes to the same string” Error

The “Received: serializes to the same string” error in Jest typically arises when you are comparing two values, and although they might appear different in your code or during debugging, Jest’s internal comparison mechanism, particularly its pretty-format module, evaluates them as identical after converting them to a string representation. This often happens with Jest’s .toEqual() matcher or when performing snapshot tests, which rely on serialized output for comparison. Jest’s toEqual() matcher performs a deep equality check, but its internal serialization process can sometimes mask subtle differences between objects, especially when those differences don’t affect their string representation.

For example, if you’re comparing two objects where one has a property set to undefined and the other has the same property missing entirely, pretty-format might serialize both to the same string. Similarly, differences in function references (e.g., two different function instances that do the same thing) or subtle variations in object properties that are not enumerable or are symbols might be ignored during serialization. This behavior is a core aspect of how Jest simplifies complex data structures for human readability and consistent comparisons, but it can lead to confusion when your expectations about object comparison differ from Jest’s internal logic. The pretty-format library is designed to make complex data structures readable, but this can sometimes hide the very differences you’re trying to assert.

To effectively troubleshoot the “Received: serializes to the same string” error, it’s vital to recognize that Jest’s primary goal with .toEqual() and snapshot tests is to compare the “value” of objects rather than their strict identity or memory reference. This deep equality check is powerful, but it’s important to be aware of its limitations and how certain data types or structural differences might be normalized during serialization. According to Jest’s official documentation, .toEqual() is designed to recursively check every field of an object or array, ensuring that they are equivalent in value, but this recursive check relies on the serialized output for presenting differences, which is where the “serializes to the same string” message originates.

Common Scenarios Leading to This Error

This perplexing Jest error often manifests in specific scenarios, primarily when dealing with complex data structures or certain types of object properties that Jest’s default serialization doesn’t differentiate. One common scenario involves comparing objects where a property is undefined in one object, and completely absent in the other. For instance, { a: 1, b: undefined } and { a: 1 } might serialize to the same string, leading to the error when using toEqual() or toMatchSnapshot(). Jest’s pretty-format often omits undefined properties during serialization, making these objects appear identical.

Another frequent cause is when comparing instances of classes or objects with non-enumerable properties, symbols, or functions. If two objects have different instances of a class, but their enumerable properties are identical, Jest might report them as the same after serialization. Similarly, functions are often serialized as [Function], meaning two different function instances will appear identical in the serialized output if their string representations are the same. This can be particularly problematic in tests involving Redux states, API responses, or database models, where objects might contain intricate structures including functions, dates, or class instances that are not straightforward to compare.

Moreover, the error can appear in snapshot testing when a slight, non-visible change occurs in the component’s output. For example, if a component’s prop changes from null to undefined, or vice versa, and this doesn’t alter the rendered DOM’s string representation, a snapshot test might throw this error. While the internal object may have changed, the serialized output, which is what the snapshot compares, remains the same. This highlights the need for careful consideration of what aspects of an object you are truly trying to assert against, and whether snapshot testing is the most appropriate method for deeply nested or volatile data structures. In such cases, a more precise Jest matcher might be necessary to pinpoint the exact difference causing the test failure.

When faced with the “Received: serializes to the same string” error, the first step in debugging is to identify the actual difference between the “received” and “expected” values. Jest’s default error message is unhelpful because it states they are the same after serialization. You need to inspect the raw objects. The most straightforward approach is to use console.log() or a debugger to print out the full contents of both the received and expected values just before the assertion fails. This allows you to see the exact structure and values of the objects, including any undefined properties, functions, or other subtle differences that Jest’s serializer might hide.

Once you’ve identified the discrepancy, you can choose the appropriate Jest matcher to assert the specific difference you care about. Here’s a breakdown of useful matchers:

1. Use .toStrictEqual(): If you need to ensure that two objects are identical in type, structure, and value, including undefined properties, .toStrictEqual() is your best friend. Unlike .toEqual(), .toStrictEqual() differentiates between undefined and missing properties, and it also checks for identical prototypes. This is often the most direct fix for the “serializes to the same string” error when you truly expect deep equality.
2. Use .toBe() for Primitives: For primitive values (numbers, strings, booleans, symbols, null, undefined), .toBe() is the correct matcher for strict equality. While less relevant for object serialization issues, it’s good practice to use it where appropriate.
3. Use .toMatchObject() for Partial Matching: If you only care that a received object contains a subset of properties with specific values, .toMatchObject() can be very useful. This allows you to ignore extraneous properties that might be causing serialization issues or are irrelevant to your test. It’s particularly helpful when testing API responses where you only need to validate certain fields.
4. Custom Serializers: For highly specific comparison needs, Jest allows you to create custom serializers. These can be used to control how certain data types or objects are transformed into strings before comparison, giving you fine-grained control over the serialization process and making the output more meaningful for your specific tests. This is an advanced technique but powerful for complex scenarios.

Remember that the goal is not just to make the test pass, but to ensure it accurately reflects the intended behavior of your code. Sometimes, the “Received: serializes to the same string” error indicates a fundamental misunderstanding of the data you are working with or an overly broad assertion. By carefully inspecting the values and choosing the most appropriate matcher, you can resolve this issue and write more precise tests that truly validate your application’s logic. For more advanced debugging techniques in JavaScript, you might find resources on effective debugging strategies helpful.

Best Practices to Avoid the Error

Preventing the “Received: serializes to the same string” error involves adopting thoughtful testing practices and a deeper understanding of Jest’s comparison mechanisms. One critical best practice is to be explicit about what you are testing. Instead of using a broad .toEqual() on large, complex objects, consider asserting specific properties or subsets of the object that are relevant to your test case. If you only care about the name and email fields of a user object, assert those directly using expect(user.name).toBe('John') and expect(user.email).toBe('john@example.com'), rather than expect(user).toEqual({ name: 'John', email: 'john@example.com', / ...other potentially problematic props / }). This reduces the surface area for unexpected serialization differences.

When dealing with objects that might contain dynamic or volatile properties (like timestamps, unique IDs, or function references), consider using Jest’s asymmetric matchers. For instance, expect.any(String) or expect.arrayContaining() can validate the type or presence of a property without requiring an exact match for its value. This is especially useful in snapshot testing, where you can replace specific parts of the snapshot with asymmetric matchers to ignore transient data. For example, if you have an object with a Question & Answer :

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']);