JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr1 = [1,2,3,4,5];
var arr2 = [4,5,6,7,8,9,10,12];

var inter = intersect(arr1,arr2);
alert(inter);


function intersect(arr1,arr2){
    //We need to know which array is the shortest to avoid useless loops
    if(arr2.length<arr1.length){
        var temp = arr1;
        arr1 = arr2;
        arr2 = temp;
    }
    // Now, we are sure arr1 is the shortest
    var result = [];
    for(var i = 0; i<arr1.length; i++){
        if(arr2.indexOf(arr1[i])>-1) result.push(arr1[i]);
    }
    return result;
}