# Intersection of two arrays JavaScript

JavascriptWeb DevelopmentObject Oriented Programming

We have two arrays of Numbers, and we are required to write a function, say intersection() that computes their intersection and returns an array that contains the intersecting elements in any order. Each element in the result should appear as many times as it shows in both arrays.

For example − If,

Input: arr1 = [1,2,3,1], arr2 = [1,3,1]
Output: [1,3,1]

## Approach

Had the arrays been sorted, we could have used the two pointer approach with initially both pointing to 0 the start of the respective array and we could have proceeded with increasing the corresponding pointer and that would have been O(m+n) complex w.r.t. time where m and n are the sizes of array.

But since we have unsorted arrays there is no logic in sorting the arrays and then using this approach, we will check every value of first against the second and construct an intersection array. This would cost us O(n^2) time.

And the code for doing this will be −

## Example

const arr1 = [1, 2, 43, 5, 3, 7, 7,8, 4, 2];
const arr2 = [1, 1, 6, 6, 2, 78, 7, 2, 3, 7, 23, 5, 3];
const intersection = (arr1, arr2) => {
const res = [];
const { length: len1 } = arr1;
const { length: len2 } = arr2;
const smaller = (len1 < len2 ? arr1 : arr2).slice();
const bigger = (len1 >= len2 ? arr1 : arr2).slice();
for(let i = 0; i < smaller.length; i++){
if(bigger.indexOf(smaller[i]) !== -1){
res.push(smaller[i]);
bigger.splice(bigger.indexOf(smaller[i]), 1, undefined);
}
};
return res;
};
console.log(intersection(arr1 ,arr2));

## Output

The output in the console will be −

[1, 2, 5, 3, 7, 7, 2]