How to Perform an SQL-like JOIN on Two JavaScript Arrays?

How to Combine Two JavaScript Arrays Based on a Common Key, Similar to an SQL JOIN

When dealing with data in JavaScript, you may encounter situations where you have multiple arrays that contain related information, and you want to combine them based on a common attribute. In this context, an SQL JOIN operation can be useful for merging data from multiple tables, and we can achieve a similar functionality in JavaScript using array manipulation techniques.

Let's say we have two arrays: userProfiles and questions. userProfiles contains objects with id and name properties, while questions contains objects with id, text, and createdBy properties. The createdBy property in questions corresponds to the id value in userProfiles.

Our goal is to "join" these arrays to create a new array that contains objects with the id, text, and name properties. In SQL, this operation would be represented by the following query:

SELECT u.id, q.text, u.name
FROM userProfiles u
JOIN questions q ON q.createdBy=u.id

In JavaScript, we can implement a join using the reduce() and concat() methods. Here's an example using the innerJoin function:

const innerJoin = (xs, ys, sel) =>
    xs.reduce((zs, x) =>
    ys.reduce((zs, y) =>        // cartesian product - all combinations
    zs.concat(sel(x, y) || []), // filter out the rows and columns you want
    zs), []);

Using this function, we can perform the join as follows:

const result = innerJoin(userProfiles, questions,
    ({id: uid, name}, {id, text, createdBy}) =>
        createdBy === uid && {id, text, name});

This innerJoin function essentially performs a Cartesian product of the two arrays, and for each pair of objects, it checks if the createdBy property in questions matches the id property in userProfiles. If they match, it returns an object with the desired properties.

The resulting result array will contain objects in the following format:

{ id, text, name }

This approach allows you to combine data from multiple arrays based on a common key, similar to an SQL JOIN operation. By leveraging JavaScript's array manipulation capabilities, you can achieve efficient and flexible data transformations for your applications.

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