retrieve the in common items in two arrays

Question

we have a social media app, and each user has a number of people whom he's following, we represent it in an array of id's [userID1, userID2, userID2..userID900], and we have another array that represent all of the users who are online on the platform, [user1, user2...user1043], the goal is to retrieve all of the online users that a particular user is following, in other words we wanna take the common items in the two arrays, Now, I know that this can easily done with a nested loop, but I want a more optimised solution.

Answer 1

*Edit: Probably the best performance you'll get is to convert the second array to either an object or a Map

const a1 = [1,2,3,4,5];
const a2 = [3,5,7,9];

const o2 = a2.reduce((a,o)=> { a[o] = true; return a; }, {});
const usingObj = a1.filter(x=> !o2[x]);
console.log(usingObj);

const m2 = new Map();
a2.forEach(x=> m2.set(x, true));

const usingMap = a1.filter(x=> !m2.has(x));
console.log(usingMap);

--- Original answer ---

You could use a combination of filter and indexOf, like this:

const a1 = [1,2,3,4,5];
const a2 = [3,5,7,9];
const a3 = a1.filter(x=> a2.indexOf(x)<0);
console.log(a3);

This will remove all items from a1 which also exist in a2.

*Edit: you could alternatively use includes instead of indexOf, as mentioned by @ColeHenderson. I tend to use indexOf because includes is relatively new. (Old dogs, new tricks) Code coverage isn't really much of a concern any more, includes has ~96% browser support

Answer 2

Lodash js recommended

var presents = _.intersectionWith(array1, array2, _.isEqual);

var dif = _.differenceWith(array1, array2, _.isEqual);

_.differenceWith is only available since 4.0.0 lodash version