Merge 2 arrays - Javascript

merge-two-arr.js

merge_array(array1, array2) {
    const result_array = [];
    const arr = array1.concat(array2);
    let len = arr.length;
    const assoc = {};

    while (len--) {
      const item = arr[len];

      if (!assoc[item]) {
        result_array.unshift(item);
        assoc[item] = true;
      }
    }
    return result_array;
}