# JavaScript Function to get the Intersect of 2 Arrays

In mathematics, the (denoted as ∩) of two sets A and B is the set that contains all elements of A that also belong to B.

I needed the javascript array equivalent, so that given an array A = [“a”,”b”,”c”, “d”] and B = [“b”, “d”, “e”], getIntersect(A, B) = [“b”, “d”]

 123456789101112 function getIntersect(arr1, arr2) {     var temp = [];     for(var i = 0; i < arr1.length; i++){         for(var k = 0; k < arr2.length; k++){             if(arr1[i] == arr2[k]){                 temp.push( arr1[i]);                 break;             }         }     }     return temp; }

### Update:

In the comments, Jeffery suggests a version using a javascript hash table (also called an associative array or a map), which would be much faster especially if arr2 was much shorter than arr1. I will do some benchmarking with some live data (from the app I extracted this from) to see how badly my version does. (See below : he sunk me big time! use his version.)

 1234567891011121314 function getIntersect(arr1, arr2) {     var r = [], o = {}, l = arr2.length, i, v;     for (i = 0; i < l; i++) {         o[arr2[i]] = true;     }     l = arr1.length;     for (i = 0; i < l; i++) {         v = arr1[i];         if (v in o) {             r.push(v);         }     }     return r; }

He is also too kind to point out one inefficient / bad practice in the code above : that I should have stored the lengths of the arrays in a variable, rather than recalculating it again and again. Nevermind …

And elsewhere, Pete pointed out the Underscore, a utility-belt library for JavaScript that provides a lot of the functional programming support, and its _.intersect function .

### Further update:

Now with benchmark results and less smoke up my ass! (repeat after me : “test your assumptions before making an ass of yourself (again)”, now repeat 100 times).

I created the (very) simple benchmarking file that would actual work, a html file with embedded javascript with has the 2 functions for testing and 2 arrays : one with elements 4646 and one with 1103 elements.

The file is http://www.falsepositives.com/files/test_array_interect_file.html. There is also a version which iterates the test 100 tests, and shows the average result in a table as below.

I also rewrote the test from using firefox’s console.log and console.time functions to using document.write and some raw javascript to log the time it took for a given function to run so I could run this in different browsers. (more about this later)

So the raw results where that Jeffery’s version blew my version out of the water! in FF 3.3 mine ran ~ 85 to 90 ms and his ~1 to 2 ms !! So creating the hash and searching it are really really optimized.

The second surprise was that my suggested optimization of my version (“storing the lengths of the arrays in a variable”) added several ms of run time! I think this is because a) the lengths are already calculated and stored as a property of each array, and b) their is some overhead of storing the 2 length values as local variables. Since object properties are really hashs, and we have just learned that hashs are really really fast the result make some sense. Storing a value instead of recalculating it is still a “really good idea”, just not this time.

Having rewritten the test in more browser generic fashion here are the results for different browsers (times are in milliseconds) :

getIntersect of a1 , a2 getIntersect of a2 , a1 getIntersect JT of a1 , a2 getIntersect JT of a2 , a1
FireFox 3.5.5 89.27 74.52 1.19 1.44
Safari 4.0.3 52.32 43.60 3.11 2.97
Safari on iPdod Touch (3.1.2) 2049.00 1712.00 48.00 39.00
Chrome 4.0.223 63.91 53.00 0.82 1.45
IE 7.0.5730 3656.00 3068.60 9.60 9.40

Your mileage will vary. The absolute number are not as interesting as the relative values.

I have also run my test against Underscore, and its _.intersect function for some interesting results:

 FireFox 232.75 Safari 137.22 Chrome 42.26 IE 1953

So not as fast as even my humble version, but keep in mind that the Underscore _.intersect function handls any number of arrays to intersect, and has 50 additional utility functions.

So what can we learn from this?

Hashes are very fast; Jeffery is smart; IE stinks; Ian needs to test his ASSumptions!

## 2 Replies to “JavaScript Function to get the Intersect of 2 Arrays”

1. Here’s how I would do it (assuming both arrays do not contain nulls or undefines):

 1234567891011121314 function getIntersect(arr1, arr2) {     var r = [], o = {}, l = arr2.length, i, v;     for (i = 0; i < l; i++) {         o[arr2[i]] = true;     }     l = arr1.length;     for (i = 0; i < l; i++) {         v = arr1[i];         if (v in o) {             r.push(v);         }     }     return r; }