javascript检查另一个数组中是否包含1个数组的元素

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.

Answer 1

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.

Answer 2

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])) 

Answer 3

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).