Joins in Javascript

Question

I have 2 lists of objects:

people = 
[{id: 1, name: \"Tom\", carid: 1},
 {id: 2, name: \"Bob\", carid: 1},
 {id: 3, name: \"Sir Benjamin Rogan-Josh IV\", car         


        

Answer 1

It is not hard to implement using underscore.js

function leftJoin(left, right, left_id, right_id) {
    var result = [];
    _.each(left, function (litem) {
        var f = _.filter(right, function (ritem) {
            return ritem[right_id] == litem[left_id];
        });
        if (f.length == 0) {
            f = [{}];
        }
        _.each(f, function (i) {
            var newObj = {};
            _.each(litem, function (v, k) {
                newObj[k + "1"] = v;
            });
            _.each(i, function (v, k) {
                newObj[k + "2"] = v;
            });
            result.push(newObj);
        });
    });
    return result;
}

leftJoin(people, cars, "carid", "id");

Answer 2

Here's a simple loop I did for a Javascript (JQuery in this case) to "join" obj1 and obj2 on someID and add one property from obj2 to obj1.

If you want to do a more complete join, you can go through and expand it to loop on obj2.hasOwnProperty() and copy that over as well.

    $.each(obj1,function(i){
        $.each(obj2, function(k){
            if (obj2[k].someID == obj1[i].someID ){
                obj1[i].someValue = obj2[k].someValue;
            }
        });
     });

Answer 3

No, LoDash does not have join it's prety easy to implement your own though, this isn't quite a join but selects all people with a matching car:

    var peopleWithCars = _.filter(people, function (person) {
        return _.exists(cars, function(car) {
            return car.id === person.id;
        });
    });

Answer 4

This implementation uses the ES6 spread operator. Again, not a library function as asked for.

const leftJoin = (objArr1, objArr2, key1, key2) => objArr1.map(
    anObj1 => ({
        ...objArr2.find(
            anObj2 => anObj1[key1] === anObj2[key2]
        ),
        ...anObj1
    })
);

Answer 5

You can do such stuff in plain javascript.

people.map(man => 
        cars.some(car => car.id === man.carid) ? 
            cars.filter(car => car.id === man.carid).map(car => ({car, man})) : 
            {man}
        ).reduce((a,b)=> a.concat(b),[]);

Answer 6

You can use Alasql JavaScript SQL library to join two or more arrays of objects:

var res = alasql('SELECT people.name AS person_name, cars.name, cars.color \
    FROM ? people LEFT JOIN ? cars ON people.carid = cars.id',[people, cars]);

Try this example at jsFiddle.