group by nested array property in JavaScript

Question

I have a json object as below in my web application. It's an array of product objects and each product object has a category property which contains an array of categories that the product belongs to.

var products = [  
   {  
      "id":1,
      "name":"Product 1",
      "price":10,
      "category":[  
         {  
            "id":10,
            "name":"Category 1"
         },
         {  
            "id":20,
            "name":"Category 2"
         }
      ]
   },
   {  
      "id":2,
      "name":"Product 2",
      "price":20,
      "category":[  
         {  
            "id":20,
            "name":"Category 2"
         },
         {  
            "id":30,
            "name":"Category 3"
         }
      ]
   }
]

So now I want to display them grouped by categories so the end result will look like below. I am already using Underscore.js in my project so it will be good if I can use it to achieve this.

var categories = [  
   {  
      "id":10,
      "name":"Category 1",
      "products":[  
         {  
            "id":1,
            "name":"Product 1",
            "price":10
         }
      ]
   },
   {  
      "id":20,
      "name":"Category 2",
      "products":[  
         {  
            "id":1,
            "name":"Product 1",
            "price":10
         }, 
         {
            "id":2,
            "name":"Product 2",
            "price":20,
         }
      ]
   },
   {  
      "id":30,
      "name":"Category 3",
      "products":[  
         {  
            "id":2,
            "name":"Product 2",
            "price":20,
         }
      ]
   }
]

Answer 1

here is what I would do

var categories = [];
var cat = new Map();

var addUniqueCategory(category) { /* determine if category is already in list of categories, if not add it to categories */ };

products.each (function (item) {
     item.categories.each(function (c) {
         if (!cat.has(c.name)) cat.set(c.name, []);

         var list = cat.get(c.name);
         list.push( { id: item.id, name: item.name, price: item.price });

         addUniqueCategory(c);
     });
});

categories.each( function (c) {
    var list = cat.get(c.name);
    if (!c.products) c.products = [];
    c.products.splice( c.length, 0, list);
 });

roughly, I'm on a phone

Answer 2

I'm not entirely sure whether there is an out-of-the-box solution to this problem with underscore, however solving this by hand shouldn't be too hard, either:

var categoriesIndexed = {};
var categories = [];

products.forEach(function(product) {
    product.category.forEach(function(category) {
        // create a new category if it does not exist yet
        if(!categoriesIndexed[category.id]) {
            categoriesIndexed[category.id] = {
                id: category.id,
                name: category.name,
                products: []
            };
            categories.push(categoriesIndexed[category.id]);
        }

        // add the product to the category
        categoriesIndexed[category.id].products.push({
            id: product.id,
            name: product.name,
            price: product.price   
        });
    });
});