How to merge two objects and calculate the average of their values in Javascript?

Question

I'm learning TDD and I would like to create a function to expected this result.

//What I expect

[
    {book_id: 1, name:'NameBook', average: x},
    {book_id: 2, name:'NameBook', average: x},
    {book_id: 3, name:'NameBook', average: x},
    {book_id: 4, name:'NameBook', average: x},
    {book_id: 5, name:'NameBook', average: x},
    {book_id: 6, name:'NameBook', average: x},
    {book_id: 7, name:'NameBook', average: x},
]

I got to merge two objects with the filter function but I'm having difficulties calculating the average and sum of these values where the id's are equal.

I'm posting here what I got to do https://jsfiddle.net/thiagolmoraes/5xsk7Lgw/1/

Anyone can help me.

 const getBooks = () => { return [ { id: 1, name: 'Python Data Science' }, { id: 2, name: 'Python Machine Learning' }, { id: 3, name: 'Development Flask' }, { id: 4, name: 'Mongo Database' }, { id: 5, name: 'ULM for Dummies' }, { id: 6, name: 'Java for Dummies' }, { id: 7, name: 'Learn Rust in 2 hours' }, ] } const getRating = () => { return [ { book_id: 1, client_id: 1, rating: 4.5 }, { book_id: 1, client_id: 4, rating: 5 }, { book_id: 1, client_id: 25, rating: 5 }, { book_id: 1, client_id: 2112, rating: 4 }, { book_id: 2, client_id: 34, rating: 3 }, { book_id: 2, client_id: 123, rating: 4 }, { book_id: 2, client_id: 23, rating: 4 }, { book_id: 2, client_id: 255, rating: 4 }, { book_id: 3, client_id: 98, rating: 2 }, { book_id: 3, client_id: 45, rating: 1 }, { book_id: 3, client_id: 223, rating: 3 }, { book_id: 3, client_id: 213, rating: 1.5 }, { book_id: 4, client_id: 652, rating: 4.5 }, { book_id: 4, client_id: 42, rating: 4.5 }, { book_id: 4, client_id: 562, rating: 4.5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, ] } const getBooksWithRatings = () => { return getBooks().map((book) => { book.ratings = getRating().filter((r) => r.book_id === book.id); return book; }); }; console.log(getBooksWithRatings())

Answer 1

It seems like you were almost there. You just need to calculate the sum of the ratings array and then divide by its length. This should complete the task for you.

 const getBooks = () => { return [ { id: 1, name: "Python Data Science" }, { id: 2, name: "Python Machine Learning" }, { id: 3, name: "Development Flask" }, { id: 4, name: "Mongo Database" }, { id: 5, name: "ULM for Dummies" }, { id: 6, name: "Java for Dummies" }, { id: 7, name: "Learn Rust in 2 hours" }, ]; }; const getRating = () => { return [ { book_id: 1, client_id: 1, rating: 4.5 }, { book_id: 1, client_id: 4, rating: 5 }, { book_id: 1, client_id: 25, rating: 5 }, { book_id: 1, client_id: 2112, rating: 4 }, { book_id: 2, client_id: 34, rating: 3 }, { book_id: 2, client_id: 123, rating: 4 }, { book_id: 2, client_id: 23, rating: 4 }, { book_id: 2, client_id: 255, rating: 4 }, { book_id: 3, client_id: 98, rating: 2 }, { book_id: 3, client_id: 45, rating: 1 }, { book_id: 3, client_id: 223, rating: 3 }, { book_id: 3, client_id: 213, rating: 1.5 }, { book_id: 4, client_id: 652, rating: 4.5 }, { book_id: 4, client_id: 42, rating: 4.5 }, { book_id: 4, client_id: 562, rating: 4.5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, ]; }; const getBooksWithRatings = () => { return getBooks().map((book) => { const ratings = getRating().filter((r) => r.book_id === book.id); book.average = ratings.reduce((acc, el) => acc + el.rating, 0) / ratings.length; return book; }); }; console.log(getBooksWithRatings());

Answer 2

You can use Array#reduce to find the sum of the filtered array and divide by its length to find the average. To prevent modifying the objects in the original array, you can use spread syntax.

 const getBooks = () => { return [ { id: 1, name: 'Python Data Science' }, { id: 2, name: 'Python Machine Learning' }, { id: 3, name: 'Development Flask' }, { id: 4, name: 'Mongo Database' }, { id: 5, name: 'ULM for Dummies' }, { id: 6, name: 'Java for Dummies' }, { id: 7, name: 'Learn Rust in 2 hours' }, ] } const getRating = () => { return [ { book_id: 1, client_id: 1, rating: 4.5 }, { book_id: 1, client_id: 4, rating: 5 }, { book_id: 1, client_id: 25, rating: 5 }, { book_id: 1, client_id: 2112, rating: 4 }, { book_id: 2, client_id: 34, rating: 3 }, { book_id: 2, client_id: 123, rating: 4 }, { book_id: 2, client_id: 23, rating: 4 }, { book_id: 2, client_id: 255, rating: 4 }, { book_id: 3, client_id: 98, rating: 2 }, { book_id: 3, client_id: 45, rating: 1 }, { book_id: 3, client_id: 223, rating: 3 }, { book_id: 3, client_id: 213, rating: 1.5 }, { book_id: 4, client_id: 652, rating: 4.5 }, { book_id: 4, client_id: 42, rating: 4.5 }, { book_id: 4, client_id: 562, rating: 4.5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 5, client_id: 2, rating: 5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 6, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, { book_id: 7, client_id: 2, rating: 4.5 }, ] } const getBooksWithRatings = () => { return getBooks().map(book => { const ratings = getRating().filter((r) => r.book_id === book.id); return {...book, average: ratings.reduce((acc,{rating})=>acc+rating, 0) / ratings.length}; }); }; console.log(getBooksWithRatings())