Simplest code for array intersection in javascript

Question

What's the simplest, library-free code for implementing array intersections in javascript? I want to write

intersection([1,2,3], [2,3,4,5]) 

and get

[2, 3] 

Answer 1

Use a combination of Array.prototype.filter and Array.prototype.includes:

const filteredArray = array1.filter(value => array2.includes(value)); 

For older browsers, with Array.prototype.indexOf and without an arrow function:

var filteredArray = array1.filter(function(n) {     return array2.indexOf(n) !== -1; }); 

NB! Both .includes and .indexOf internally compares elements in the array by using ===, so if the array contains objects it will only compare object references (not their content). If you want to specify your own comparison logic, use Array.prototype.some instead.

Answer 2

Destructive seems simplest, especially if we can assume the input is sorted:

/* destructively finds the intersection of   * two arrays in a simple fashion.    *  * PARAMS  *  a - first array, must already be sorted  *  b - second array, must already be sorted  *  * NOTES  *  State of input arrays is undefined when  *  the function returns.  They should be   *  (prolly) be dumped.  *  *  Should have O(n) operations, where n is   *    n = MIN(a.length, b.length)  */ function intersection_destructive(a, b) {   var result = [];   while( a.length > 0 && b.length > 0 )   {        if      (a[0] < b[0] ){ a.shift(); }      else if (a[0] > b[0] ){ b.shift(); }      else /* they're equal */      {        result.push(a.shift());        b.shift();      }   }    return result; } 

Non-destructive has to be a hair more complicated, since we’ve got to track indices:

/* finds the intersection of   * two arrays in a simple fashion.    *  * PARAMS  *  a - first array, must already be sorted  *  b - second array, must already be sorted  *  * NOTES  *  *  Should have O(n) operations, where n is   *    n = MIN(a.length(), b.length())  */ function intersect_safe(a, b) {   var ai=0, bi=0;   var result = [];    while( ai < a.length && bi < b.length )   {      if      (a[ai] < b[bi] ){ ai++; }      else if (a[ai] > b[bi] ){ bi++; }      else /* they're equal */      {        result.push(a[ai]);        ai++;        bi++;      }   }    return result; }