javascript check existence of elements of 1 array inside the other

Question

For searching elements of one array inside the other, one may use the indexOf() on the target of search and run a loop on the other array elements and in each step check existence. This is trivial and my knowledge on Javascript is too. Could any one please suggest a more efficient way? Maybe even a built-in method of the language could help? Although I couldn't hit to such a method using google.

`indexOf` is probably as efficient as you need, but there are some other methods mentioned at http://stackoverflow.com/questions/1885557/simplest-code-for-array-intersection-in-javascript and possibly at http://stackoverflow.com/questions/11076067/finding-matches-between-multiple-javascript-arrays — Rhumborl, Sep 24 '14 at 07:58
You have to use `indexOf` or a loop for a reliable cross browser solution. There are methods available in frameworks like jQuery (e.g: `inArray`), but that also make use of the same. — Diode, Sep 24 '14 at 08:01

score 2 · Accepted Answer · edited Sep 24 '14 at 08:35

You can use Array.filter() internally and implement a function on Array's prototype which returns elements which are common to both.

Array.prototype.common = function(a) {
    return this.filter(function(i) {
      return a.indexOf(i) >= 0;
    });
};


alert([1,2,3,4,5].common([4,5,6])); // "4, 5"

Again as you mention in your post, this logic also works by taking each element and checking whether it exists in the other.

georg · Answer 2 · 2014-09-24T08:48:14.183

A hair more efficient way is convert one of the arrays into a hash table and then loop through the second one, checking the presence of elements at O(1) time:

a = [1,2,3,4,5]
b = [1,7,3,8,5]

map = {}
a.forEach(function(x) { map[x] = 1 })
intersection = b.filter(function(x) { return map[x] === 1 })

document.write(JSON.stringify(intersection))

This only works if elements in question are primitives. For arrays of objects you have to resort to the indexOf method.

ES6 ("Harmony") does support Set, but strangely not set operations (union, intersection etc), so these should be coded by hand:

// Firefox only

a = [1,2,3,4,5]
b = [1,7,3,8,5]

sa = new Set(a)
sb = new Set(b)

sa.forEach(function(x) {
  if (!sb.has(x))
    sa.delete(x);
});

document.write(uneval([...sa]))

score 0 · Answer 3 · answered Sep 24 '14 at 07:59

0

JavaScript's arrays don't have a method that does intersections. Various libraries provide methods to do it (by looping the array as you describe), including Underscore and PrototypeJS (and others, I'm sure).

answered Sep 24 '14 at 07:59

T.J. Crowder

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javascript check existence of elements of 1 array inside the other

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