How to do groupBy and add the sum of the values grouped in underscore.js - react-native

Question

From this:
var x = [
{category: 'Title', price: 100 },
{category: 'Title', price: 200 },
{category: 'Title3', price: 300 },
{category: 'Title3', price: 400 },
{category: 'Title5', price: 500 },
];
To this:
var x = [
{category: 'Title', price: 300},
{category: 'Title3', price: 700},
{category: 'Title5', price: 500},
];
The logic is same as SQL query:
SELECT category, sum (price) FROM x GROUP BY category;

Answer 1

Updated:
groupBy will group the initial array by "category".
map will then iterate each object in the grouped array by passing in two params: key and value. The first key will be "Title" and its value is an array of [{category: 'Title', price: 100 }, {category: 'Title', price: 200 }].
Before returning an updated array in map, reduce is used to sum up the "price". Basically, it iterates through the array (first param) and return the sum from the function (second param). Note that in the first iteration, the total will be 0 (last param in reduce). So, the first return will be 0 + 100 which is then passed in as 'new' total in the second iteration and so on.
You can refer to the documentation for more details.
Try this:
var groups = _.groupBy(x, 'category');
var result = _.map(groups, function(value, key) {
return {
category: key,
price: _.reduce(value, function(total, o) {
return total + o.price;
}, 0)
};
});