Intersect arrays in Javascript

Question

How can I INTERSECT N arrays in Javascript at a reasonably fast time?

Ie.

arr1 = [1,2,3,4,5];
arr2 = [1,2,3,4];
arr3 = [4];
arr4 = [4,5];

Result: [4]

rather than revinvent the wheel, check this out: https://stackoverflow.com/questions/37320296/how-to-calculate-intersection-of-multiple-arrays-in-javascript-and-what-does-e — james emanon, Jul 14 '18 at 18:13
In the future, if you mean to respond to a specific person's comment, then use put @username in your comment. That's the ONLY way they will get notified that you've done something you want them to see. Otherwise, they'll never see your comment. — jfriend00, Jul 15 '18 at 23:04

Nina Scholz · Answer 1 · 2018-07-14T18:17:20.983

3

You could take an intersection function for common elements with a Set and Array#filter.

function common(a, b) {
    return b.filter(Set.prototype.has.bind(new Set(a)));
}

console.log([[1, 2, 3, 4, 5], [1, 2, 3, 4], [4], [4, 5]].reduce(common));

edited Jul 14 '18 at 18:17

answered Jul 14 '18 at 18:15

Nina Scholz

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Is this "reasonably fast" as the OP asked? It creates a `new Set()` a lot. – jfriend00 Jul 14 '18 at 18:19
for the given data, it is fast to write ... o_O – Nina Scholz Jul 14 '18 at 18:20
Nina, can you explain this bit: Set.prototype.has.bind(new Set(a)) – james emanon Jul 14 '18 at 18:49
1

@jamesemanon, what i want, is to take a function as callback. in this case `has` from `Set`, but it is not possible to use `new Set([...]).has`, because it returns an error `Set.prototype.has: 'this' is not a Set object`. therefore i took the prototype and with `bind` it takes a set as `this`. voila – Nina Scholz Jul 14 '18 at 18:56
imho, that is pretty clever – james emanon Jul 14 '18 at 19:02

Mark · Answer 2 · 2018-07-14T18:56:08.253

I would simply use filter() and includes() unless I knew my arrays were going to be very large. This runs "reasonably fast" and is probably faster than creating a bunch of sets when you will be joining a lot of normal-sized arrays, however for fewer, very large arrays, the new Set() could prove faster :

let arr1 = [1,2,3,4,5];
let arr2 = [1,2,3,4,5];
let arr3 = [4];
let arr4 = [4,5];

let r = [arr1, arr2, arr3, arr4].reduce((a, c) => a.filter(i => c.includes(i)))
console.log(r)

Intersect arrays in Javascript

2 Answers2