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Mastering Jest: Understanding and Conquering Common Errors

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

As a developer, errors are a part and parcel of the job. One such error you might come across while using Jest for testing your JavaScript code is the “Received: serializes to the same string” error.

This error message might seem cryptic at first glance, but the root cause of the problem is actually straightforward.

Comparison using toBe() method

To understand the “Received: serializes to the same string” error, let’s first talk about the toBe() method in Jest. This method is used to check if two values are the same, i.e., if they have the same reference in memory.

However, this method only works for primitive data types like numbers, strings, and booleans. When dealing with non-primitive values like objects and arrays, the toBe() method will fail even if the values have the same properties or elements.

Solution using toStrictEqual() method

To solve this problem, we can use the toStrictEqual() method instead of toBe(). The toStrictEqual() method performs a deep comparison between two values, i.e., it checks that the objects or arrays have the same keys/properties or elements, respectively, and that those keys/properties or elements have the same values.

Differences between toStrictEqual() and toEqual() methods

You might be wondering what the difference between toStrictEqual() and the toEqual() method is. While both methods perform a deep comparison, the toEqual() method only checks for value equality.

In contrast, the toStrictEqual() method also checks for the type equality of the object or array. Solution using JSON.stringify() method

Another way to compare literal values in Jest is by using the JSON.stringify() method.

This method converts any object or array into a string that can be compared to another string. However, this method only works for literal values, i.e., values that can be written in the code such as strings, numbers, and arrays that only contain literal values.

Checking for referential identity using toBe() method

It’s important to note that when using the toBe() method, it checks for referential identity, i.e., it checks if the two variables being compared point to the same object in memory. This can be useful in some cases, but when dealing with non-primitive values, it’s better to use the toStrictEqual() method for a more accurate comparison.

Avoiding static property on array or object

One common mistake that can lead to the “Received: serializes to the same string” error is having a static property on an object or array. A static property is a property that is shared among all instances of an object or array.

If we try to compare two objects or arrays with the same static property, they will appear equal to the toBe() method even if they have different values. To avoid this issue, we should avoid using static properties on objects and arrays.

Checking if an object matches a subset of properties of another object

Finally, when comparing objects, we might only be interested in specific properties of each object rather than checking for all properties. In this case, we can use the toMatchObject() method, which checks if an object has all the properties and values of another object.

However, the toMatchObject() method does not check for extra properties in the tested object.

Using toStrictEqual() Method for Comparison of non-primitive values

Now that we understand the differences between the toStrictEqual() and toEqual() methods, let’s dive deeper into the toStrictEqual() method.

Definition and working of toStrictEqual() method

The toStrictEqual() method is used to compare non-primitive values like objects and arrays. It checks that two objects have the same keys and values, or that two arrays have the same elements in the same order.

The toStrictEqual() method also performs a type check to ensure that both objects or arrays have the same type.

Comparison of objects and arrays using toStrictEqual()

Let’s look at an example to compare two objects using toStrictEqual():

“`

const obj1 = { name: “John”, age: 30 }

const obj2 = { age: 30, name: “John” }

expect(obj1).toStrictEqual(obj2)

“`

In this example, we have declared two objects `obj1` and `obj2` with the same keys and values. However, the order of the keys is different in each object.

If we were to use the toEqual() method, the test would fail because the order of the keys is different. However, by using the toStrictEqual() method, the test passes because both objects have the same keys and values.

Differences between toStrictEqual() and toEqual()

To summarize, the main difference between toStrictEqual() and toEqual() is that toStrictEqual() performs a type check in addition to a deep comparison of the values. Additionally, toStrictEqual() is more specific and highlights differences in the types of values being compared.

On the other hand, toEqual() only checks for value equality and does not take into account the types of the values being compared.

Conclusion

We hope this article has helped you understand the Jest error “Received: serializes to the same string” and how to use the toStrictEqual() method for comparing non-primitive values. By understanding the differences between toStrictEqual() and other comparison methods like toEqual() and toBe(), you can write more accurate and effective tests for your JavaScript code.

Remember to avoid static properties on objects and arrays and use toMatchObject() to compare a subset of properties when necessary. Using the JSON.stringify() Method for Comparison of Non-primitive Values

In addition to the toStrictEqual() and toEqual() methods, we can also use the JSON.stringify() method to compare non-primitive values.

This method converts a JavaScript object or array into a JSON string, which allows us to compare non-primitive values as if they were primitive values. Definition and working of JSON.stringify() Method

The JSON.stringify() method is a built-in JavaScript function that takes an object or array as an argument and returns a JSON string.

The syntax is as follows:

“`

JSON.stringify(value[, replacer[, space]])

“`

The `value` argument is the object or array to be converted to a JSON string. The optional `replacer` argument can be used to filter out unwanted properties, and the optional `space` argument can be used to add indentation to the output.

To compare two non-primitive values using JSON.stringify(), we first convert both values to JSON strings and then compare the strings using the toEqual() method. Comparison using JSON.stringify() and toEqual() Methods

Let’s look at an example to compare two objects using JSON.stringify():

“`

const obj1 = { name: “John”, age: 30 }

const obj2 = { age: 30, name: “John” }

expect(JSON.stringify(obj1)).toEqual(JSON.stringify(obj2))

“`

In this example, we’re using JSON.stringify() to convert both objects `obj1` and `obj2` into JSON strings before we compare them with toEqual().

By doing this, we’re comparing the objects as if they were primitive values, which is the intended behavior with toEqual(). However, it’s important to note that using JSON.stringify() has some limitations.

It only works for literal values that can be written in the code, such as strings, numbers, and arrays that only contain literal values. In addition, using JSON.stringify() can be less efficient than using the toStrictEqual() or toEqual() methods, especially for large objects and arrays.

The toEqual() Method for Comparison of Non-primitive Values

The toEqual() method is a powerful comparison method in Jest that allows us to compare non-primitive values with the same ease as primitive values. Let’s take a closer look at this method and how it works.

Definition and working of toEqual() Method

The toEqual() method is a Jest matcher that compares two values for equality, even if they are non-primitive values like objects or arrays. The matcher performs a deep comparison of the two values, checking that all properties and values match.

This means that the toEqual() method is more specific than the toBe() method, which only checks if the two values are the same reference in memory.

Comparison using toEqual() Method

Let’s look at an example to compare two objects using toEqual():

“`

const obj1 = { name: “John”, age: 30 }

const obj2 = { age: 30, name: “John” }

expect(obj1).toEqual(obj2)

“`

In this example, we’re using the toEqual() method to compare two objects `obj1` and `obj2`, even though the order of the keys is different in each object. The toEqual() method performs a deep comparison of the objects’ properties and values, which is why the test passes even though the properties are in a different order.

One potential issue with the toEqual() method is that it can be less precise than the toStrictEqual() method, which checks for type equality in addition to value equality. However, in most cases, the toEqual() method is sufficient and more readable than using JSON.stringify().

Conclusion

In this article, we’ve covered two additional methods for comparing non-primitive values in Jest: JSON.stringify() and toEqual(). By understanding the pros and cons of these methods and when to use them, you can write more accurate and effective tests for your JavaScript code.

Whether you’re using toStrictEqual(), toEqual(), or JSON.stringify(), the important thing is to make sure your tests accurately reflect the behavior of your code and catch any bugs before they make it into production.

Checking for Referential Identity Using the toBe() Method

In Jest, the toBe() method is commonly used to check if two values are equal. However, it’s important to understand that this method compares referential identity, not value identity.

Let’s dive deeper into what that means and how to use the toBe() method effectively.

Definition and Working of toBe() Method

The toBe() method is a Jest matcher that checks whether two values have the same reference in memory, i.e., whether they are the same object. This works well for primitive data types like numbers and strings, as they have a unique reference in memory.

However, when dealing with non-primitive data types like objects and arrays, the toBe() method will fail even if the values are structurally the same.

Comparison using toBe() Method

Let’s look at an example to compare two objects using toBe():

“`

const obj1 = { name: “John”, age: 30 }

const obj2 = { name: “John”, age: 30 }

expect(obj1).toBe(obj2)

“`

In this example, we’re using the toBe() method to compare two objects `obj1` and `obj2`. Even though both objects have the same properties and values, the test fails because they have different references in memory.

As a general rule, when comparing non-primitive data types, we should use the toEqual() or toStrictEqual() methods instead of the toBe() method. These methods perform a deep comparison of the values, checking for value equality instead of referential identity.

Avoiding Static Property on Array or Object

Static properties in JavaScript are properties that are shared among all instances of an object or array. When dealing with arrays in JavaScript, it’s important to note that JavaScript does not support negative indexing, i.e., indexing an array from the end.

Trying to access an element with a negative index will result in an undefined value.

Overview of Javascript Not Supporting Negative Indexing

For example, let’s say we have an array `arr = [1, 2, 3, 4]` and we want to access the last element in the array. In some other programming languages, we can do this by using negative indexing, e.g., `arr[-1]`.

However, in JavaScript, this will result in an undefined value.

Checking for Static Property on Array

One way static properties can cause issues when comparing arrays is by adding properties to the Array.prototype object. If we add a property to the Array.prototype object, it will be shared among all arrays in our code, which can lead to unintended side effects.

For example, let’s say we add a property to the Array.prototype object like this:

“`

Array.prototype.sum = function() {

return this.reduce((acc, val) => acc + val)

}

“`

Now, if we create two arrays in our code, e.g., `arr1 = [1, 2, 3]` and `arr2 = [4, 5, 6]`, and compare them using toBe(), the test will fail because both arrays now have the sum() property. To avoid this issue, we should avoid modifying the Array.prototype object directly and instead create a new object with the properties we need and use it when creating new arrays.

Checking for Static Property on Object

Similarly, we can run into issues with static properties when dealing with objects. If we add a static property to an object, it will be shared among all instances of that object.

For example, let’s say we create two instances of an object like this:

“`

function Person(name) {

this.name = name

}

const person1 = new Person(“John”)

const person2 = new Person(“Jane”)

Person.prototype.count = 0

“`

Now, if we compare `person1` and `person2` using toBe(), the test will fail because both objects have the count property. To avoid this issue, we should avoid adding static properties to objects and instead use instance properties or methods.

Conclusion

By avoiding static properties on objects and arrays and using the appropriate comparison method for non-primitive data types, we can write accurate and effective tests in Jest. Additionally, by understanding the nuances of negative indexing in JavaScript, we can avoid unintended side effects and ensure the correctness of our code.

Checking if an Object Matches Subset of Properties of Another Object

In some cases, we might only be interested in comparing a subset of properties between two objects. The toMatchObject() method in Jest enables us to check if an object has all the properties and values of another object, without needing to check for every property.

Definition and Working of toMatchObject() Method

The toMatchObject() method is a Jest matcher that checks if an object has all the properties and values of another object. The matcher performs a shallow comparison of the object’s properties and values, i.e., it doesn’t compare the references of any nested objects or arrays.

This is different from the toEqual() method, which performs a deep comparison of the object’s properties and values.

Comparison using toMatchObject() Method

Let’s look at an example to compare two objects using toMatchObject():

“`

const obj1 = { name: “John”, age: 30, city: “New York” }

const obj2 = { name: “John”, age: 30 }

expect(obj1).not.toMatchObject(obj2)

“`

In this example, we’re using the toMatchObject() method to check if the `obj1` object has all the properties and values of the `obj2` object. Since `obj2` doesn’t have the `city` property, the test will fail and return false.

By using the to

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