Simplest code for array intersection in javascript

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]

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.

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;
}

If your environment supports ECMAScript 6 Set, one simple and supposedly efficient (see specification link) way:

function intersect(a, b) {
  var setA = new Set(a);
  var setB = new Set(b);
  var intersection = new Set([...setA].filter(x => setB.has(x)));
  return Array.from(intersection);
}

Shorter, but less readable (also without creating the additional intersection Set):

function intersect(a, b) {
  var setB = new Set(b);
  return [...new Set(a)].filter(x => setB.has(x));
}

Note that when using sets you will only get distinct values, thus new Set([1, 2, 3, 3]).size evaluates to 3.

Using Underscore.js or lodash.js

_.intersection( [0,345,324] , [1,0,324] )  // gives [0,324]

// Return elements of array a that are also in b in linear time:
function intersect(a, b) {
  return a.filter(Set.prototype.has, new Set(b));
}

// Example:
console.log(intersect([1,2,3], [2,3,4,5]));

I recommend above succinct solution which outperforms other implementations on large inputs. If performance on small inputs matters, check the alternatives below.

Alternatives and performance comparison:

See the following snippet for alternative implementations and check https://jsperf.com/array-intersection-comparison for performance comparisons.

function intersect_for(a, b) {
  const result = [];
  const alen = a.length;
  const blen = b.length;
  for (let i = 0; i < alen; ++i) {
    const ai = a[i];
    for (let j = 0; j < blen; ++j) {
      if (ai === b[j]) {
        result.push(ai);
        break;
      }
    }
  } 
  return result;
}

function intersect_filter_indexOf(a, b) {
  return a.filter(el => b.indexOf(el) !== -1);
}

function intersect_filter_in(a, b) {
  const map = b.reduce((map, el) => {map[el] = true; return map}, {});
  return a.filter(el => el in map);
}

function intersect_for_in(a, b) {
  const result = [];
  const map = {};
  for (let i = 0, length = b.length; i < length; ++i) {
    map[b[i]] = true;
  }
  for (let i = 0, length = a.length; i < length; ++i) {
    if (a[i] in map) result.push(a[i]);
  }
  return result;
}

function intersect_filter_includes(a, b) {
  return a.filter(el => b.includes(el));
}

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

function intersect_filter_has_arrow(a, b) {
  const set = new Set(b);
  return a.filter(el => set.has(el));
}

function intersect_for_has(a, b) {
  const result = [];
  const set = new Set(b);
  for (let i = 0, length = a.length; i < length; ++i) {
    if (set.has(a[i])) result.push(a[i]);
  }
  return result;
}

Results in Firefox 53:

• Ops/sec on large arrays (10,000 elements):

   filter + has (this)               523 (this answer)
   for + has                         482
   for-loop + in                     279
   filter + in                       242
   for-loops                          24
   filter + includes                  14
   filter + indexOf                   10
  

• Ops/sec on small arrays (100 elements):

   for-loop + in                 384,426
   filter + in                   192,066
   for-loops                     159,137
   filter + includes             104,068
   filter + indexOf               71,598
   filter + has (this)            43,531 (this answer)
   filter + has (arrow function)  35,588
  

My contribution in ES6 terms. In general it finds the intersection of an array with indefinite number of arrays provided as arguments.

Array.prototype.intersect = function(...a) {
  return [this,...a].reduce((p,c) => p.filter(e => c.includes(e)));
}
var arrs = [[0,2,4,6,8],[4,5,6,7],[4,6]],
     arr = [0,1,2,3,4,5,6,7,8,9];

document.write("<pre>" + JSON.stringify(arr.intersect(...arrs)) + "</pre>");

How about just using associative arrays?

function intersect(a, b) {
    var d1 = {};
    var d2 = {};
    var results = [];
    for (var i = 0; i < a.length; i++) {
        d1[a[i]] = true;
    }
    for (var j = 0; j < b.length; j++) {
        d2[b[j]] = true;
    }
    for (var k in d1) {
        if (d2[k]) 
            results.push(k);
    }
    return results;
}

edit:

// new version
function intersect(a, b) {
    var d = {};
    var results = [];
    for (var i = 0; i < b.length; i++) {
        d[b[i]] = true;
    }
    for (var j = 0; j < a.length; j++) {
        if (d[a[j]]) 
            results.push(a[j]);
    }
    return results;
}

The performance of @atk's implementation for sorted arrays of primitives can be improved by using .pop rather than .shift.

function intersect(array1, array2) {
   var result = [];
   // Don't destroy the original arrays
   var a = array1.slice(0);
   var b = array2.slice(0);
   var aLast = a.length - 1;
   var bLast = b.length - 1;
   while (aLast >= 0 && bLast >= 0) {
      if (a[aLast] > b[bLast] ) {
         a.pop();
         aLast--;
      } else if (a[aLast] < b[bLast] ){
         b.pop();
         bLast--;
      } else /* they're equal */ {
         result.push(a.pop());
         b.pop();
         aLast--;
         bLast--;
      }
   }
   return result;
}

I created a benchmark using jsPerf: http://bit.ly/P9FrZK. It's about three times faster to use .pop.

1. Sort it
2. check one by one from the index 0, create new array from that.

Something like this, Not tested well though.

function intersection(x,y){
 x.sort();y.sort();
 var i=j=0;ret=[];
 while(i<x.length && j<y.length){
  if(x[i]<y[j])i++;
  else if(y[j]<x[i])j++;
  else {
   ret.push(x[i]);
   i++,j++;
  }
 }
 return ret;
}

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

PS:The algorithm only intended for Numbers and Normal Strings, intersection of arbitary object arrays may not work.

Using jQuery:

var a = [1,2,3];
var b = [2,3,4,5];
var c = $(b).not($(b).not(a));
alert(c);

If you need to have it handle intersecting multiple arrays:

const intersect = (a1, a2, ...rest) => {
  const a12 = a1.filter(value => a2.includes(value))
  if (rest.length === 0) { return a12; }
  return intersect(a12, ...rest);
};

console.log(intersect([1,2,3,4,5], [1,2], [1, 2, 3,4,5], [2, 10, 1])) 

For arrays containing only strings or numbers you can do something with sorting, as per some of the other answers. For the general case of arrays of arbitrary objects I don't think you can avoid doing it the long way. The following will give you the intersection of any number of arrays provided as parameters to arrayIntersection:

var arrayContains = Array.prototype.indexOf ?
    function(arr, val) {
        return arr.indexOf(val) > -1;
    } :
    function(arr, val) {
        var i = arr.length;
        while (i--) {
            if (arr[i] === val) {
                return true;
            }
        }
        return false;
    };

function arrayIntersection() {
    var val, arrayCount, firstArray, i, j, intersection = [], missing;
    var arrays = Array.prototype.slice.call(arguments); // Convert arguments into a real array

    // Search for common values
    firstArray = arrays.pop();
    if (firstArray) {
        j = firstArray.length;
        arrayCount = arrays.length;
        while (j--) {
            val = firstArray[j];
            missing = false;

            // Check val is present in each remaining array 
            i = arrayCount;
            while (!missing && i--) {
                if ( !arrayContains(arrays[i], val) ) {
                    missing = true;
                }
            }
            if (!missing) {
                intersection.push(val);
            }
        }
    }
    return intersection;
}

arrayIntersection( [1, 2, 3, "a"], [1, "a", 2], ["a", 1] ); // Gives [1, "a"]; 

Another indexed approach able to process any number of arrays at once:

// Calculate intersection of multiple array or object values.
function intersect (arrList) {
    var arrLength = Object.keys(arrList).length;
        // (Also accepts regular objects as input)
    var index = {};
    for (var i in arrList) {
        for (var j in arrList[i]) {
            var v = arrList[i][j];
            if (index[v] === undefined) index[v] = 0;
            index[v]++;
        };
    };
    var retv = [];
    for (var i in index) {
        if (index[i] == arrLength) retv.push(i);
    };
    return retv;
};

It works only for values that can be evaluated as strings and you should pass them as an array like:

intersect ([arr1, arr2, arr3...]);

...but it transparently accepts objects as parameter or as any of the elements to be intersected (always returning array of common values). Examples:

intersect ({foo: [1, 2, 3, 4], bar: {a: 2, j:4}}); // [2, 4]
intersect ([{x: "hello", y: "world"}, ["hello", "user"]]); // ["hello"]
 

EDIT: I just noticed that this is, in a way, slightly buggy.

That is: I coded it thinking that input arrays cannot itself contain repetitions (as provided example doesn't).

But if input arrays happen to contain repetitions, that would produce wrong results. Example (using below implementation):

intersect ([[1, 3, 4, 6, 3], [1, 8, 99]]);
// Expected: [ '1' ]
// Actual: [ '1', '3' ]

Fortunately this is easy to fix by simply adding second level indexing. That is:

Change:

        if (index[v] === undefined) index[v] = 0;
        index[v]++;

by:

        if (index[v] === undefined) index[v] = {};
        index[v][i] = true; // Mark as present in i input.

...and:

         if (index[i] == arrLength) retv.push(i);

by:

         if (Object.keys(index[i]).length == arrLength) retv.push(i);

Complete example:

// Calculate intersection of multiple array or object values.
function intersect (arrList) {
    var arrLength = Object.keys(arrList).length;
        // (Also accepts regular objects as input)
    var index = {};
    for (var i in arrList) {
        for (var j in arrList[i]) {
            var v = arrList[i][j];
            if (index[v] === undefined) index[v] = {};
            index[v][i] = true; // Mark as present in i input.
        };
    };
    var retv = [];
    for (var i in index) {
        if (Object.keys(index[i]).length == arrLength) retv.push(i);
    };
    return retv;
};

intersect ([[1, 3, 4, 6, 3], [1, 8, 99]]); // [ '1' ]

A tiny tweak to the smallest one here (the filter/indexOf solution), namely creating an index of the values in one of the arrays using a JavaScript object, will reduce it from O(N*M) to "probably" linear time. source1 source2

function intersect(a, b) {
  var aa = {};
  a.forEach(function(v) { aa[v]=1; });
  return b.filter(function(v) { return v in aa; });
}

This isn't the very simplest solution (it's more code than filter+indexOf), nor is it the very fastest (probably slower by a constant factor than intersect_safe()), but seems like a pretty good balance. It is on the very simple side, while providing good performance, and it doesn't require pre-sorted inputs.

With some restrictions on your data, you can do it in linear time!

For positive integers: use an array mapping the values to a "seen/not seen" boolean.

function intersectIntegers(array1,array2) { 
   var seen=[],
       result=[];
   for (var i = 0; i < array1.length; i++) {
     seen[array1[i]] = true;
   }
   for (var i = 0; i < array2.length; i++) {
     if ( seen[array2[i]])
        result.push(array2[i]);
   }
   return result;
}

There is a similar technique for objects: take a dummy key, set it to "true" for each element in array1, then look for this key in elements of array2. Clean up when you're done.

function intersectObjects(array1,array2) { 
   var result=[];
   var key="tmpKey_intersect"
   for (var i = 0; i < array1.length; i++) {
     array1[i][key] = true;
   }
   for (var i = 0; i < array2.length; i++) {
     if (array2[i][key])
        result.push(array2[i]);
   }
   for (var i = 0; i < array1.length; i++) {
     delete array1[i][key];
   }
   return result;
}

Of course you need to be sure the key didn't appear before, otherwise you'll be destroying your data...

function intersection(A,B){
var result = new Array();
for (i=0; i<A.length; i++) {
	for (j=0; j<B.length; j++) {
		if (A[i] == B[j] && $.inArray(A[i],result) == -1) {
			result.push(A[i]);
		}
	}
}
return result;
}

For simplicity:

// Usage
const intersection = allLists
  .reduce(intersect, allValues)
  .reduce(removeDuplicates, []);


// Implementation
const intersect = (intersection, list) =>
  intersection.filter(item =>
    list.some(x => x === item));

const removeDuplicates = (uniques, item) =>
  uniques.includes(item) ? uniques : uniques.concat(item);


// Example Data
const somePeople = [bob, doug, jill];
const otherPeople = [sarah, bob, jill];
const morePeople = [jack, jill];

const allPeople = [...somePeople, ...otherPeople, ...morePeople];
const allGroups = [somePeople, otherPeople, morePeople];

// Example Usage
const intersection = allGroups
  .reduce(intersect, allPeople)
  .reduce(removeDuplicates, []);

intersection; // [jill]

Benefits:

• dirt simple
• data-centric
• works for arbitrary number of lists
• works for arbitrary lengths of lists
• works for arbitrary types of values
• works for arbitrary sort order
• retains shape (order of first appearance in any array)
• exits early where possible
• memory safe, short of tampering with Function / Array prototypes

Drawbacks:

• higher memory usage
• higher CPU usage
• requires an understanding of reduce
• requires understanding of data flow

You wouldn't want to use this for 3D engine or kernel work, but if you have problems getting this to run in an event-based app, your design has bigger problems.

I'll contribute with what has been working out best for me:

if (!Array.prototype.intersect){
Array.prototype.intersect = function (arr1) {

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

A functional approach with ES2015

A functional approach must consider using only pure functions without side effects, each of which is only concerned with a single job.

These restrictions enhance the composability and reusability of the functions involved.

// small, reusable auxiliary functions

const createSet = xs => new Set(xs);
const filter = f => xs => xs.filter(apply(f));
const apply = f => x => f(x);


// intersection

const intersect = xs => ys => {
  const zs = createSet(ys);
  return filter(x => zs.has(x)
     ? true
     : false
  ) (xs);
};


// mock data

const xs = [1,2,2,3,4,5];
const ys = [0,1,2,3,3,3,6,7,8,9];


// run it

console.log( intersect(xs) (ys) );

Please note that the native Set type is used, which has an advantageous lookup performance.

Avoid duplicates

Obviously repeatedly occurring items from the first Array are preserved, while the second Array is de-duplicated. This may be or may be not the desired behavior. If you need a unique result just apply dedupe to the first argument:

// auxiliary functions

const apply = f => x => f(x);
const comp = f => g => x => f(g(x));
const afrom = apply(Array.from);
const createSet = xs => new Set(xs);
const filter = f => xs => xs.filter(apply(f));


// intersection

const intersect = xs => ys => {
  const zs = createSet(ys);
  return filter(x => zs.has(x)
     ? true
     : false
  ) (xs);
};


// de-duplication

const dedupe = comp(afrom) (createSet);


// mock data

const xs = [1,2,2,3,4,5];
const ys = [0,1,2,3,3,3,6,7,8,9];


// unique result

console.log( intersect(dedupe(xs)) (ys) );

Compute the intersection of any number of Arrays

If you want to compute the intersection of an arbitrarily number of Arrays just compose intersect with foldl. Here is a convenience function:

// auxiliary functions

const apply = f => x => f(x);
const uncurry = f => (x, y) => f(x) (y);
const createSet = xs => new Set(xs);
const filter = f => xs => xs.filter(apply(f));
const foldl = f => acc => xs => xs.reduce(uncurry(f), acc);


// intersection

const intersect = xs => ys => {
  const zs = createSet(ys);
  return filter(x => zs.has(x)
     ? true
     : false
  ) (xs);
};


// intersection of an arbitrarily number of Arrays

const intersectn = (head, ...tail) => foldl(intersect) (head) (tail);


// mock data

const xs = [1,2,2,3,4,5];
const ys = [0,1,2,3,3,3,6,7,8,9];
const zs = [0,1,2,3,4,5,6];


// run

console.log( intersectn(xs, ys, zs) );

.reduce to build a map, and .filter to find the intersection. delete within the .filter allows us to treat the second array as though it's a unique set.

function intersection (a, b) {
  var seen = a.reduce(function (h, k) {
    h[k] = true;
    return h;
  }, {});

  return b.filter(function (k) {
    var exists = seen[k];
    delete seen[k];
    return exists;
  });
}

I find this approach pretty easy to reason about. It performs in constant time.

I have written an intesection function which can even detect intersection of array of objects based on particular property of those objects.

For instance,

if arr1 = [{id: 10}, {id: 20}]
and arr2 =  [{id: 20}, {id: 25}]

and we want intersection based on the id property, then the output should be :

[{id: 20}]

As such, the function for the same (note: ES6 code) is :

const intersect = (arr1, arr2, accessors = [v => v, v => v]) => {
    const [fn1, fn2] = accessors;
    const set = new Set(arr2.map(v => fn2(v)));
    return arr1.filter(value => set.has(fn1(value)));
};

and you can call the function as:

intersect(arr1, arr2, [elem => elem.id, elem => elem.id])

Also note: this function finds intersection considering the first array is the primary array and thus the intersection result will be that of the primary array.

Here is underscore.js implementation:

_.intersection = function(array) {
  if (array == null) return [];
  var result = [];
  var argsLength = arguments.length;
  for (var i = 0, length = array.length; i < length; i++) {
    var item = array[i];
    if (_.contains(result, item)) continue;
    for (var j = 1; j < argsLength; j++) {
      if (!_.contains(arguments[j], item)) break;
    }
    if (j === argsLength) result.push(item);
  }
  return result;
};

Source: http://underscorejs.org/docs/underscore.html#section-62

Create an Object using one array and loop through the second array to check if the value exists as key.

function intersection(arr1, arr2) {
  var myObj = {};
  var myArr = [];
  for (var i = 0, len = arr1.length; i < len; i += 1) {
    if(myObj[arr1[i]]) {
      myObj[arr1[i]] += 1; 
    } else {
      myObj[arr1[i]] = 1;
    }
  }
  for (var j = 0, len = arr2.length; j < len; j += 1) {
    if(myObj[arr2[j]] && myArr.indexOf(arr2[j]) === -1) {
      myArr.push(arr2[j]);
    }
  }
  return myArr;
}

This function avoids the N^2 problem, taking advantage of the power of dictionaries. Loops through each array only once, and a third and shorter loop to return the final result. It also supports numbers, strings, and objects.

function array_intersect(array1, array2) 
{
    var mergedElems = {},
        result = [];

    // Returns a unique reference string for the type and value of the element
    function generateStrKey(elem) {
        var typeOfElem = typeof elem;
        if (typeOfElem === 'object') {
            typeOfElem += Object.prototype.toString.call(elem);
        }
        return [typeOfElem, elem.toString(), JSON.stringify(elem)].join('__');
    }

    array1.forEach(function(elem) {
        var key = generateStrKey(elem);
        if (!(key in mergedElems)) {
            mergedElems[key] = {elem: elem, inArray2: false};
        }
    });

    array2.forEach(function(elem) {
        var key = generateStrKey(elem);
        if (key in mergedElems) {
            mergedElems[key].inArray2 = true;
        }
    });

    Object.values(mergedElems).forEach(function(elem) {
        if (elem.inArray2) {
            result.push(elem.elem);
        }
    });

    return result;
}

If there is a special case that cannot be solved, just by modifying the generateStrKey function, it could surely be solved. The trick of this function is that it uniquely represents each different data according to type and value.

---

This variant has some performance improvements. Avoid loops in case any array is empty. It also starts by walking through the shorter array first, so if it finds all the values of the first array in the second array, exits the loop.

function array_intersect(array1, array2) 
{
    var mergedElems = {},
        result = [],
        firstArray, secondArray,
        firstN = 0, 
        secondN = 0;

    function generateStrKey(elem) {
        var typeOfElem = typeof elem;
        if (typeOfElem === 'object') {
            typeOfElem += Object.prototype.toString.call(elem);
        }
        return [typeOfElem, elem.toString(), JSON.stringify(elem)].join('__');
    }

    // Executes the loops only if both arrays have values
    if (array1.length && array2.length) 
    {
        // Begins with the shortest array to optimize the algorithm
        if (array1.length < array2.length) {
            firstArray = array1;
            secondArray = array2;
        } else {
            firstArray = array2;
            secondArray = array1;            
        }

        firstArray.forEach(function(elem) {
            var key = generateStrKey(elem);
            if (!(key in mergedElems)) {
                mergedElems[key] = {elem: elem, inArray2: false};
                // Increases the counter of unique values in the first array
                firstN++;
            }
        });

        secondArray.some(function(elem) {
            var key = generateStrKey(elem);
            if (key in mergedElems) {
                if (!mergedElems[key].inArray2) {
                    mergedElems[key].inArray2 = true;
                    // Increases the counter of matches
                    secondN++;
                    // If all elements of first array have coincidence, then exits the loop
                    return (secondN === firstN);
                }
            }
        });

        Object.values(mergedElems).forEach(function(elem) {
            if (elem.inArray2) {
                result.push(elem.elem);
            }
        });
    }

    return result;
}

I think using an object internally can help with computations and could be performant too.

// Approach maintains a count of each element and works for negative elements too

function intersect(a,b){
    
    const A = {};
    a.forEach((v)=>{A[v] ? ++A[v] : A[v] = 1});
    const B = {};
    b.forEach((v)=>{B[v] ? ++B[v] : B[v] = 1});
    const C = {};
    Object.entries(A).map((x)=>C[x[0]] = Math.min(x[1],B[x[0]]))
    return Object.entries(C).map((x)=>Array(x[1]).fill(Number(x[0]))).flat();
}
const x = [1,1,-1,-1,0,0,2,2];
const y = [2,0,1,1,1,1,0,-1,-1,-1];
const result = intersect(x,y);
console.log(result);  // (7) [0, 0, 1, 1, 2, -1, -1]

I am using map even object could be used.

//find intersection of 2 arrs
const intersections = (arr1,arr2) => {
  let arrf = arr1.concat(arr2)
  let map = new Map();
  let union = [];
  for(let i=0; i<arrf.length; i++){
    if(map.get(arrf[i])){
      map.set(arrf[i],false);
    }else{
      map.set(arrf[i],true);
    }
  }
 map.forEach((v,k)=>{if(!v){union.push(k);}})
 return union;
}

I extended tarulen's answer to work with any number of arrays. It also should work with non-integer values.

function intersect() { 
    const last = arguments.length - 1;
    var seen={};
    var result=[];
    for (var i = 0; i < last; i++)   {
        for (var j = 0; j < arguments[i].length; j++)  {
            if (seen[arguments[i][j]])  {
                seen[arguments[i][j]] += 1;
            }
            else if (!i)    {
                seen[arguments[i][j]] = 1;
            }
        }
    }
    for (var i = 0; i < arguments[last].length; i++) {
        if ( seen[arguments[last][i]] === last)
            result.push(arguments[last][i]);
        }
    return result;
}

If your arrays are sorted, this should run in O(n), where n is min( a.length, b.length )

function intersect_1d( a, b ){
    var out=[], ai=0, bi=0, acurr, bcurr, last=Number.MIN_SAFE_INTEGER;
    while( ( acurr=a[ai] )!==undefined && ( bcurr=b[bi] )!==undefined ){
        if( acurr < bcurr){
            if( last===acurr ){
                out.push( acurr );
            }
            last=acurr;
            ai++;
        }
        else if( acurr > bcurr){
            if( last===bcurr ){
                out.push( bcurr );
            }
            last=bcurr;
            bi++;
        }
        else {
            out.push( acurr );
            last=acurr;
            ai++;
            bi++;
        }
    }
    return out;
}

var arrays = [
    [1, 2, 3],
    [2, 3, 4, 5]
]
function commonValue (...arr) {
    let res = arr[0].filter(function (x) {
        return arr.every((y) => y.includes(x))
    })
    return res;
}
commonValue(...arrays);

function intersectionOfArrays(arr1, arr2) {
    return arr1.filter((element) => arr2.indexOf(element) !== -1).filter((element, pos, self) => self.indexOf(element) == pos);
}

This is a modern and simple ES6 way to do it that is also very flexible. It lets you specify multiple arrays as the arrays to compare the subject array to, and can work in both inclusive and exclusive mode.

// =======================================
// The function
// =======================================

function assoc(subjectArray, otherArrays, { mustBeInAll = true } = {}) {
  return subjectArray.filter((subjectItem) => {
    if (mustBeInAll) {
      return otherArrays.every((otherArray) =>
        otherArray.includes(subjectItem)
      );
    } else {
      return otherArrays.some((otherArray) => otherArray.includes(subjectItem));
    }
  });
}

// =======================================
// The usage
// =======================================

const cheeseList = ["stilton", "edam", "cheddar", "brie"];
const foodListCollection = [
  ["cakes", "ham", "stilton"],
  ["juice", "wine", "brie", "bread", "stilton"]
];

// Output will be: ['stilton', 'brie']
const inclusive = assoc(cheeseList, foodListCollection, { mustBeInAll: false }),

// Output will be: ['stilton']
const exclusive = assoc(cheeseList, foodListCollection, { mustBeInAll: true })

Live example: https://codesandbox.io/s/zealous-butterfly-h7dgf?fontsize=14&hidenavigation=1&theme=dark

Here's a very naive implementation I'm using. It's non-destructive and also makes sure not to duplicate entires.

Array.prototype.contains = function(elem) {
	return(this.indexOf(elem) > -1);
};

Array.prototype.intersect = function( array ) {
	// this is naive--could use some optimization
	var result = [];
	for ( var i = 0; i < this.length; i++ ) {
		if ( array.contains(this[i]) && !result.contains(this[i]) )
			result.push( this[i] );
	}
	return result;
}

intersection of N arrays in coffeescript

getIntersection: (arrays) ->
    if not arrays.length
        return []
    a1 = arrays[0]
    for a2 in arrays.slice(1)
        a = (val for val in a1 when val in a2)
        a1 = a
    return a1.unique()

Building on Anon's excellent answer, this one returns the intersection of two or more arrays.

function arrayIntersect(arrayOfArrays)
{        
    var arrayCopy = arrayOfArrays.slice(),
        baseArray = arrayCopy.pop();
    
    return baseArray.filter(function(item) {
        return arrayCopy.every(function(itemList) {
            return itemList.indexOf(item) !== -1;
        });
    });
}

Hope this Helps for all versions.

function diffArray(arr1, arr2) {
  var newArr = [];
  
  var large = arr1.length>=arr2.length?arr1:arr2;
  var small = JSON.stringify(large) == JSON.stringify(arr1)?arr2:arr1;
  for(var i=0;i<large.length;i++){
    var copyExists = false; 
    for(var j =0;j<small.length;j++){
      if(large[i]==small[j]){
        copyExists= true;
        break;
      }
    }
    if(!copyExists)
      {
        newArr.push(large[i]);
      }
  }
  
  for(var i=0;i<small.length;i++){
    var copyExists = false; 
    for(var j =0;j<large.length;j++){
      if(large[j]==small[i]){
        copyExists= true;
        break;
      }
    }
    if(!copyExists)
      {
        newArr.push(small[i]);
      }
  }
  
  
  return newArr;
}

not about efficiency, but easy to follow, here is an example of unions and intersections of sets, it handles arrays of sets and sets of sets.

http://jsfiddle.net/zhulien/NF68T/

// process array [element, element...], if allow abort ignore the result
function processArray(arr_a, cb_a, blnAllowAbort_a)
{
	var arrResult = [];
	var blnAborted = false;
	var intI = 0;

	while ((intI < arr_a.length) && (blnAborted === false))
	{
		if (blnAllowAbort_a)
		{
			blnAborted = cb_a(arr_a[intI]);
		}
		else
		{
			arrResult[intI] = cb_a(arr_a[intI]);
		}
		intI++;
	}
	
	return arrResult;
}

// process array of operations [operation,arguments...]
function processOperations(arrOperations_a)
{
	var arrResult = [];
	var fnOperationE;

	for(var intI = 0, intR = 0; intI < arrOperations_a.length; intI+=2, intR++) 
	{
		var fnOperation = arrOperations_a[intI+0];
		var fnArgs = arrOperations_a[intI+1];
		if (fnArgs === undefined)
		{
			arrResult[intR] = fnOperation();
		}
		else
		{
			arrResult[intR] = fnOperation(fnArgs);
		}
	}
	
	return arrResult;
}

// return whether an element exists in an array
function find(arr_a, varElement_a)
{
	var blnResult = false;
	
	processArray(arr_a, function(varToMatch_a)
	{
		var blnAbort = false;
		
		if (varToMatch_a === varElement_a)
		{
			blnResult = true;
			blnAbort = true;
		}
		
		return blnAbort;
	}, true);
	
	return blnResult;
}

// return the union of all sets
function union(arr_a)
{
	var arrResult = [];
	var intI = 0;
	
	processArray(arr_a, function(arrSet_a)
	{
		processArray(arrSet_a, function(varElement_a)
		{
			// if the element doesn't exist in our result
			if (find(arrResult, varElement_a) === false)
			{
				// add it
				arrResult[intI] = varElement_a;
				intI++;
			}
		});
	});
	
	return arrResult;
}

// return the intersection of all sets
function intersection(arr_a)
{
	var arrResult = [];
	var intI = 0;
	
	// for each set
	processArray(arr_a, function(arrSet_a)
	{
		// every number is a candidate
		processArray(arrSet_a, function(varCandidate_a)
		{
			var blnCandidate = true;
			
			// for each set
			processArray(arr_a, function(arrSet_a)
			{
				// check that the candidate exists
				var blnFoundPart = find(arrSet_a, varCandidate_a);
				
				// if the candidate does not exist
				if (blnFoundPart === false)
				{
					// no longer a candidate
					blnCandidate = false;
				}
			});
			
			if (blnCandidate)
			{
				// if the candidate doesn't exist in our result
				if (find(arrResult, varCandidate_a) === false)
				{
					// add it
					arrResult[intI] = varCandidate_a;
					intI++;
				}
			}
		});
	});
	
	return arrResult;
}

var strOutput = ''

var arrSet1 = [1,2,3];
var arrSet2 = [2,5,6];
var arrSet3 = [7,8,9,2];

// return the union of the sets
strOutput = union([arrSet1, arrSet2, arrSet3]);
alert(strOutput);

// return the intersection of 3 sets
strOutput = intersection([arrSet1, arrSet2, arrSet3]);
alert(strOutput);

// of 3 sets of sets, which set is the intersecting set
strOutput = processOperations([intersection,[[arrSet1, arrSet2], [arrSet2], [arrSet2, arrSet3]]]);
alert(strOutput);

"filter" and "indexOf" aren't supported on Array in IE. How about this:

var array1 = [1, 2, 3];
var array2 = [2, 3, 4, 5];

var intersection = [];
for (i in array1) {
    for (j in array2) {
        if (array1[i] == array2[j]) intersection.push(array1[i]);
    }
}