Скачать или смотреть Understanding Why JavaScript Arrays of Objects May Not Appear Equal

Discover the solution to comparing nested arrays of objects in JavaScript, even when the order of keys differs.
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Understanding Why JavaScript Arrays of Objects May Not Appear Equal

In the world of JavaScript, it’s common to handle data structures like arrays of objects, especially when working with databases. However, a common issue developers face is determining whether two arrays of objects are equal. Particularly, when retrieving data from a database like MongoDB, the returned objects may not always align with our expectations. This guide will delve into a specific problem and provide a clear solution.

The Problem

Let's break down a scenario where you may run into confusion. You have two objects: objA retrieved from MongoDB and objB from the request body. Each of these objects contains an array of nested objects treated as properties .test.

Here’s a simplified view of the objects you may be comparing:

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When you run a comparison using a function like isEqual, you find that the result is false, even though the contents of the two arrays are identical when considered as a collection of key-value pairs. This can lead to confusion and frustration when trying to ensure the returned object matches your expected data structure.

Analyzing the Comparison

At first glance, it might look like the two arrays should be equal since they hold the same key-value pairs. However, since the keys in objB are in a different order than in objA, typical comparison methods that check for strict equality fail.

Key Points:

Order Matters: In JavaScript, when comparing objects and arrays, the order of keys/properties matters — thus, even if two objects contain the same key-value pairs, they will not be considered equal if the order of those keys is different.

Array of Objects: The challenge lies in comparing arrays of objects, especially when they are retrieved from an external source like a database.

The Solution

The solution to this comparison issue relies on using the .lean() method with MongoDB's findOne(). By default, Mongoose returns a document as a Mongoose object, which includes methods and properties that could affect equality checks. By calling .lean(), you instruct Mongoose to return a plain JavaScript object, which simplifies comparisons.

Implementation Steps:

Modify Your Query: Update your MongoDB query by appending .lean():

[[See Video to Reveal this Text or Code Snippet]]

Compare the Arrays: You can use the _.isEqual() from Lodash for a straightforward comparison after ensuring both objects are plain JavaScript objects:

[[See Video to Reveal this Text or Code Snippet]]

Manual Comparison (Optional)

If you wish to construct a manual comparison function (not usually necessary but for exploratory purposes), here's a basic way to compare two objects by key-value pairs:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

In summary, the issue of two arrays of objects being perceived as unequal can often be traced back to the structure of the data being compared. By using .lean() in your MongoDB queries, you can ensure that you are working with simple, comparable JavaScript objects. Remember that when comparing arrays of objects, both structure and order matter. Happy coding!