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

Question

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.

Nina Scholz · Accepted Answer · 2016-07-08 06:41:28Z

5

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);

edited Jul 8, 2016 at 6:41

answered Jul 8, 2016 at 6:28

Nina Scholz

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2 Comments

Kwhitejr Over a year ago

Can't say that I fully understand it (yet), but it sure does work. Mahalo. Are you able to explain a bit about what is happening on line: var key = groupBy.map(function (k) { return a[k]; }).join('|');

Nina Scholz Over a year ago

this line builds a new key with the values of the properties, defined in groupBy. for example for the first element, you get for the first property type the value "MOTOR VEHICLE THEFT" and for to_timestamp the value "2015-09-24T00:00:00.000Z" in an array. this is then joined with | and the result for the key is "MOTOR VEHICLE THEFT|2015-09-24T00:00:00.000Z". this result is used as hash for the object.

孙欣乐 · Accepted Answer · 2016-07-08 06:53:05Z

but I think you reduce a object will be easier more then a array. after you got a object, you can convert it to a array

This code are just combine the data and type to the object key. Did not add district.

const object = [{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"}]

// reduce a object 
.reduce( ( res, item) => {
  
  const date = moment(item.to_timestamp).format('YYYY/MM/DD')
  const key = item.type + date
  
  if ( !res[key] ) {
    res[key] = {key: item.type, date: date, value: item.count}
  } else {
    res[key]['value'] += item.count
  }
  return res
}, {} )

//get a object, and then the object to array
//console.log(object) 
var result = []
for ( var key in object ) {
  result.push(object[key])
}
console.log(result)

<script src="http://momentjs.com/downloads/moment.min.js"></script>

Morteza Tourani · Accepted Answer · 2016-07-08 06:37:41Z

I think you can do this with a simple Array.prototype.forEach loop:

var crimes = [
	{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"}
];

var dict = {},
	result = [];
crimes.forEach(crime => {
	var date = new Date(crime.to_timestamp);
	date.setDate(date.getDate() - date.getDay());
	var hash = crime.type + date.toDateString()
	if (!dict[hash]) {
		dict[hash] = {
			count: crime.count,
			key: crime.type,
			date: date
		};
		result.push(dict[hash])
	} else
		dict[hash].count += crime.count;
});

console.log(result);

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