How Can I Perform an SQL-Style Inner Join on Two JavaScript Arrays?

Joining JavaScript Arrays with SQL-Like Syntax

Problem:

You have two JavaScript arrays: one containing user profiles and the other containing questions. The user profile array consists of objects with id and name keys, while the questions array consists of objects with id, text, and createdBy keys. The createdBy key in questions always matches an id value in user profiles.

Your goal is to "join" these arrays in a similar manner to joining tables in a database, creating a new array that contains objects with id, text, and name keys.

Solution:

JavaScript provides an efficient method for performing inner joins on arrays using the following steps:

1. Define an Inner Join Function:

   Create a function called innerJoin that takes three parameters: two arrays to be joined, xs and ys, and a selector function sel that specifies which properties to include in the joined array.

2. Cartesian Product Reduction:

   Use reduce to iterate over each element in xs and, for each element, iterate over each element in ys. This effectively creates a Cartesian product of the two arrays.

3. Filter and Select:

   Inside the nested loop, apply the selector function sel to each pair of elements. The selector function should return a new object with the desired properties. If the selector function returns a falsey value, the pair is discarded.

4. Concatenate Results:

   The innerJoin function returns an array that is the result of concatenating all the objects returned by the selector function.

Implementation:

Here's a demonstration using the following data sets:

const userProfiles = [
  {id: 1, name: "Ashok"},
  {id: 2, name: "Amit"},
  {id: 3, name: "Rajeev"},
];

const questions = [
  {id: 1, text: "text1", createdBy: 2},
  {id: 2, text: "text2", createdBy: 2},
  {id: 3, text: "text3", createdBy: 1},
  {id: 4, text: "text4", createdBy: 2},
  {id: 5, text: "text5", createdBy: 3},
  {id: 6, text: "text6", createdBy: 3},
];

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

Output:

console.log(result);
// Outputs:
[
  {id: 1, text: "text3", name: "Ashok"},
  {id: 2, text: "text1", name: "Amit"},
  {id: 2, text: "text2", name: "Amit"},
  {id: 2, text: "text4", name: "Amit"},
  {id: 3, text: "text5", name: "Rajeev"},
  {id: 3, text: "text6", name: "Rajeev"},
]

This output demonstrates that the innerJoin function has successfully joined the two arrays, selecting the id, text, and name properties.

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