How to get the difference between two arrays in JavaScript?

Question

Is there a way to return the difference between two arrays in JavaScript?

For example:

var a1 = ['a', 'b']; var a2 = ['a', 'b', 'c', 'd'];  // need ["c", "d"] 

Answer 1

There is a better way using ES7:

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Intersection

 let intersection = arr1.filter(x => arr2.includes(x)); 

For [1,2,3] [2,3] it will yield [2,3]. On the other hand, for [1,2,3] [2,3,5] will return the same thing.

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Difference

let difference = arr1.filter(x => !arr2.includes(x)); 

For [1,2,3] [2,3] it will yield [1]. On the other hand, for [1,2,3] [2,3,5] will return the same thing.

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For a symmetric difference, you can do:

let difference = arr1                  .filter(x => !arr2.includes(x))                  .concat(arr2.filter(x => !arr1.includes(x))); 

This way, you will get an array containing all the elements of arr1 that are not in arr2 and vice-versa

As @Joshaven Potter pointed out on his answer, you can add this to Array.prototype so it can be used like this:

Array.prototype.diff = function(arr2) { return this.filter(x => !arr2.includes(x)); } [1, 2, 3].diff([2, 3]) 

Answer 2

Array.prototype.diff = function(a) {     return this.filter(function(i) {return a.indexOf(i) < 0;}); };  ////////////// // Examples // //////////////  const dif1 = [1,2,3,4,5,6].diff( [3,4,5] );   console.log(dif1); // => [1, 2, 6]   const dif2 = ["test1", "test2","test3","test4","test5","test6"].diff(["test1","test2","test3","test4"]);   console.log(dif2); // => ["test5", "test6"]

Note .indexOf() and .filter() are not available before IE9.