Kwhitejr Kwhitejr - 4 months ago 12
Javascript Question

How to reduce an array of objects on multiple variables in Javascript?

I would like to know how to reduce a large array of objects based upon several properties. The array looks like:

[{count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:6, district:12, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:14, district:19, to_timestamp:"2015-10-01T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"VANDALISM"},
...
{count:4, district:19, to_timestamp:"2016-03-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:7, district:10, to_timestamp:"2016-03-24T00:00:00.000Z", type:"ASSAULT"}]


Resulting reduced array of objects needs to include the aggregate
count
from every
district
for given crime
type
on any given date (
to_timestamp
); dates are weekly. Something like:

[{key: "MOTOR VEHICLE THEFT", value: 57, date:"2015/09/23"},
...
{key: "ASSAULT", value: 77, date:"2016/03/23"}]


I'm already using Moment for date conversions.

Answer

You could use an object as hash table for the wanted groups (type and to_timestamp) and use Array#forEach for iterating the array.

var data = [{ count: 4, district: 19, to_timestamp: "2015-09-24T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 6, district: 12, to_timestamp: "2015-09-24T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 14, district: 19, to_timestamp: "2015-10-01T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 4, district: 19, to_timestamp: "2015-09-24T00:00:00.000Z", type: "VANDALISM" }, { count: 4, district: 19, to_timestamp: "2016-03-24T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 7, district: 10, to_timestamp: "2016-03-24T00:00:00.000Z", type: "ASSAULT" }],
    groupBy = ['type', 'to_timestamp'],
    grouped = [];

data.forEach(function (a) {
    var key = groupBy.map(function (k) { return a[k]; }).join('|');
    if (!this[key]) {
        this[key] = { key: a.type, value: 0, date: a.to_timestamp.slice(0, 10).replace(/-/g, '/') };
        grouped.push(this[key]);
    }
    this[key].value += a.count;
}, Object.create(null));

console.log(grouped);

Comments