Compute intersection of two arrays in JavaScript

Question

Given two arrays of unequal length:

var arr1 = ["mike", "sue", "tom", "kathy", "henry"]; //arr1.length = 5
var arr2 = ["howey", "jim", "sue", "jennifer", "kathy", "hank", "alex"]; //arr2.length = 7

How can I find the values common to both arrays? In this case "sue" and "kathy" should be returned.

score 60 · Answer 1 · edited Feb 21 '17 at 16:25

60

Here is an intersection function based on Array.prototype.filter

function intersect(a, b) {
    var t;
    if (b.length > a.length) t = b, b = a, a = t; // indexOf to loop over shorter
    return a.filter(function (e) {
        return b.indexOf(e) > -1;
    });
}

var arr1 = ["mike", "sue", "tom", "kathy", "henry"];
    arr2 = ["howey", "jim", "sue", "jennifer", "kathy", "hank", "alex"];

intersect(arr1, arr2); // ["sue", "kathy"]

You might also want to consider the following

var arr1 = ['sue', 'sue', 'kathy'],
    arr2 = ['kathy', 'kathy', 'sue'];

The above would now give ["sue", "sue", "kathy"]. If you don't want duplicates you could do a further filter on this. This would also standardise results. i.e.

return a
    .filter(/* .. */) // same as before
    .filter(function (e, i, c) { // extra step to remove duplicates
        return c.indexOf(e) === i;
    });

Adding this will now return the same result as the previous arrays (["sue", "kathy"]), even though there were duplicates.

edited Feb 21 '17 at 16:25

Kerem

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answered Apr 26 '13 at 01:24

Paul S.

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1

Thanks Paul for giving both the remove dupes and regular options, Nice bit of code. – JimTheDev Jan 26 '14 at 06:14
I dont believe this will work if one of the two arrays is empty, it will return a false positive...I simply did a check added a check for .length – afreeland Aug 05 '14 at 14:36
10

Arghh.. I hate `if (condition) return true;` construction.. Why not `return condition;`? – vp_arth Apr 14 '15 at 08:45
2

@vp_arth to make it more obvious what's going on, of course feel free to modify it if you decide to use the code – Paul S. Apr 14 '15 at 11:05
`[1]`,`[1,1]` shouldn't it return [1] ? – solimanware Feb 17 '17 at 18:38
@afreeland If one is empty, an empty array will be returned (right?). Is this not the intended behavior? – Sinjai Jul 18 '17 at 18:29
Removing duplicates from the starting array before comparing its items with others might be a bit more efficient, especially if there is a large number of duplicates. – ScampMichael May 23 '18 at 15:58

score 57 · Answer 2 · answered Apr 26 '13 at 01:16

57

You could use Array.filter:

var result = arr1.filter(function(n) {
  return arr2.indexOf(n) > -1;
});

answered Apr 26 '13 at 01:16

xdazz

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Why not just `return arr2.indexOf(n) !== -1`? You're making a new array either way. – Blender Apr 26 '13 at 01:17
1

@alex filter() returns array of matched values, while forEach does not return anything. – Nitin Jadhav Jul 07 '15 at 05:55
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This should be marked as the accepted answer – scottmgerstl Jun 05 '18 at 02:51
1

@Blender 7 years later... "Why not just..." prolly conventional C/C++ paranoia. – bvj Feb 15 '20 at 05:14

alex · Answer 3 · 2014-12-12T20:50:06.487

25

You want to find the intersection of two arrays?

You could use Underscore's intersection(). This will give you a list of values present in both arrays.

var commonValues = _.intersection(arr1, arr2);

jsFiddle.

If you didn't want to use a library, it'd be trivial to implement...

var commonValues = arr1.filter(function(value) { 
                                   return arr2.indexOf(value) > -1;
                               });

jsFiddle.

If Array.prototype.filter() and Array.prototype.indexOf() are not supported in your target platforms...

var commonValues = [];
var i, j;
var arr1Length = arr1.length;
var arr2Length = arr2.length;

for (i = 0; i < arr1Length; i++) {
    for (j = 0; j < arr2Length; j++) {
        if (arr1[i] === arr2[j]) {
            commonValues.push(arr1[i]);
        }
    }
}

jsFiddle.

edited Dec 12 '14 at 20:50

answered Apr 26 '13 at 01:15

alex

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Reading this made me start thinking.. what should the expected result of an intersection be if you have `"sue"` appearing _multiple times_ in one/both of the arrays? – Paul S. Apr 26 '13 at 01:38
@PaulS. Interesting thoughts. I guess this would add it multiple times if it were in the source array multiple times. If it were an issue, you could use a *drop duplicates* type method on the result (or factor it as you went in the above code). – alex Apr 26 '13 at 01:40

score 9 · Answer 4 · answered Apr 26 '13 at 01:17

9

Iterate over one of the arrays and compare the objects with the other:

var results = [];

for (var i = 0; i < arr1.length; i++) {
    if (arr2.indexOf(arr1[i]) !== -1) {
        results.push(arr1[i]);
    }
}

answered Apr 26 '13 at 01:17

Blender

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Compute intersection of two arrays in JavaScript

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