The most efficient method to group by a key on an array of objects in js is to use the reduce function. The reduce() method executes a reducer function (that you provide) on each element of the array, resulting in a single output value.
Use the Array. We can use the JavaScript array reduce method to combine objects in an array into one object. We have the arr array which we want to combine into one object. To do that, we call reduce with a callback that returns an object with obj spread into the returned object. And we add the item.
groupBy() groups the items into a plain JavaScript object, while array. groupByToMap() groups them into a Map instance. If you'd like to use these functions right away, then use the polyfill provided by core-js library.
To convert an object to an array you use one of three methods: Object. keys() , Object. values() , and Object. entries() .
In plain Javascript, you could use Array#reduce
with an object
var cars = [{ make: 'audi', model: 'r8', year: '2012' }, { make: 'audi', model: 'rs5', year: '2013' }, { make: 'ford', model: 'mustang', year: '2012' }, { make: 'ford', model: 'fusion', year: '2015' }, { make: 'kia', model: 'optima', year: '2012' }],
result = cars.reduce(function (r, a) {
r[a.make] = r[a.make] || [];
r[a.make].push(a);
return r;
}, Object.create(null));
console.log(result);
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Timo's answer is how I would do it. Simple _.groupBy
, and allow some duplications in the objects in the grouped structure.
However the OP also asked for the duplicate make
keys to be removed. If you wanted to go all the way:
var grouped = _.mapValues(_.groupBy(cars, 'make'),
clist => clist.map(car => _.omit(car, 'make')));
console.log(grouped);
Yields:
{ audi:
[ { model: 'r8', year: '2012' },
{ model: 'rs5', year: '2013' } ],
ford:
[ { model: 'mustang', year: '2012' },
{ model: 'fusion', year: '2015' } ],
kia:
[ { model: 'optima', year: '2012' } ]
}
If you wanted to do this using Underscore.js, note that its version of _.mapValues
is called _.mapObject
.
You are looking for _.groupBy()
.
Removing the property you are grouping by from the objects should be trivial if required:
var cars = [{'make':'audi','model':'r8','year':'2012'},{'make':'audi','model':'rs5','year':'2013'},{'make':'ford','model':'mustang','year':'2012'},{'make':'ford','model':'fusion','year':'2015'},{'make':'kia','model':'optima','year':'2012'},];
var grouped = _.groupBy(cars, function(car) {
return car.make;
});
console.log(grouped);
<script src='https://cdn.jsdelivr.net/lodash/4.17.2/lodash.min.js'></script>
As a bonus, you get even nicer syntax with ES6 arrow functions:
const grouped = _.groupBy(cars, car => car.make);
There is absolutely no reason to download a 3rd party library to achieve this simple problem, like the above solutions suggest.
The one line version to group a list
of objects by a certain key
in es6:
const groupByKey = (list, key) => list.reduce((hash, obj) => ({...hash, [obj[key]]:( hash[obj[key]] || [] ).concat(obj)}), {})
The longer version that filters out the objects without the key
:
function groupByKey(array, key) {
return array
.reduce((hash, obj) => {
if(obj[key] === undefined) return hash;
return Object.assign(hash, { [obj[key]]:( hash[obj[key]] || [] ).concat(obj)})
}, {})
}
var cars = [{'make':'audi','model':'r8','year':'2012'},{'make':'audi','model':'rs5','year':'2013'},{'make':'ford','model':'mustang','year':'2012'},{'make':'ford','model':'fusion','year':'2015'},{'make':'kia','model':'optima','year':'2012'}];
console.log(groupByKey(cars, 'make'))
NOTE: It appear the original question asks how to group cars by make, but omit the make in each group. So the short answer, without 3rd party libraries, would look like this:
const groupByKey = (list, key, {omitKey=false}) => list.reduce((hash, {[key]:value, ...rest}) => ({...hash, [value]:( hash[value] || [] ).concat(omitKey ? {...rest} : {[key]:value, ...rest})} ), {})
var cars = [{'make':'audi','model':'r8','year':'2012'},{'make':'audi','model':'rs5','year':'2013'},{'make':'ford','model':'mustang','year':'2012'},{'make':'ford','model':'fusion','year':'2015'},{'make':'kia','model':'optima','year':'2012'}];
console.log(groupByKey(cars, 'make', {omitKey:true}))
Here is your very own groupBy
function which is a generalization of the code from: https://github.com/you-dont-need/You-Dont-Need-Lodash-Underscore
function groupBy(xs, f) {
return xs.reduce((r, v, i, a, k = f(v)) => ((r[k] || (r[k] = [])).push(v), r), {});
}
const cars = [{ make: 'audi', model: 'r8', year: '2012' }, { make: 'audi', model: 'rs5', year: '2013' }, { make: 'ford', model: 'mustang', year: '2012' }, { make: 'ford', model: 'fusion', year: '2015' }, { make: 'kia', model: 'optima', year: '2012' }];
const result = groupBy(cars, (c) => c.make);
console.log(result);
var cars = [{
make: 'audi',
model: 'r8',
year: '2012'
}, {
make: 'audi',
model: 'rs5',
year: '2013'
}, {
make: 'ford',
model: 'mustang',
year: '2012'
}, {
make: 'ford',
model: 'fusion',
year: '2015'
}, {
make: 'kia',
model: 'optima',
year: '2012'
}].reduce((r, car) => {
const {
model,
year,
make
} = car;
r[make] = [...r[make] || [], {
model,
year
}];
return r;
}, {});
console.log(cars);
Its also possible with a simple for
loop:
const result = {};
for(const {make, model, year} of cars) {
if(!result[make]) result[make] = [];
result[make].push({ model, year });
}
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