// https://stackoverflow.com/a/10499519/860099
function A(arr1,arr2) {
  return _.union(arr1,arr2)
}

A(a1,a2);

// https://stackoverflow.com/a/53149853/860099
function B(arr1,arr2) {
  return _.unionWith(arr1, arr2, _.isEqual);
}

B(a1,a2);

// https://stackoverflow.com/a/27664971/860099
function C(arr1,arr2) {
  return [...new Set([...arr1,...arr2])]
}

C(a1,a2);

// https://stackoverflow.com/a/48130841/860099
function D(arr1,arr2) {
  return Array.from(new Set(arr1.concat(arr2)))
}

D(a1,a2);

// https://stackoverflow.com/a/23080662/860099
function E(arr1,arr2) {
  return arr1.concat(arr2.filter((item) => arr1.indexOf(item) < 0))
}

E(a1,a2);

// https://stackoverflow.com/a/28631880/860099
function G(arr1,arr2) {
  var hash = {};
  var i;
  
  for (i = 0; i < arr1.length; i++) {
    hash[arr1[i]] = true;
  }
  for (i = 0; i < arr2.length; i++) {
    hash[arr2[i]] = true;
  }
  return Object.keys(hash);
}

G(a1,a2);

// https://stackoverflow.com/a/13847481/860099
function H(a, b){
    var hash = {};
    var ret = [];

    for(var i=0; i < a.length; i++){
        var e = a[i];
        if (!hash[e]){
            hash[e] = true;
            ret.push(e);
        }
    }

    for(var i=0; i < b.length; i++){
        var e = b[i];
        if (!hash[e]){
            hash[e] = true;
            ret.push(e);
        }
    }

    return ret;
}

H(a1,a2);

// https://stackoverflow.com/a/1584377/860099
function J(arr1,arr2) {
  function arrayUnique(array) {
      var a = array.concat();
      for(var i=0; i<a.length; ++i) {
          for(var j=i+1; j<a.length; ++j) {
              if(a[i] === a[j])
                  a.splice(j--, 1);
          }
      }

      return a;
  }

  return arrayUnique(arr1.concat(arr2));
}

J(a1,a2);

// https://stackoverflow.com/a/25120770/860099
function L(array1, array2) {
    const array3 = array1.slice(0);
    let len1 = array1.length;
    let len2 = array2.length;
    const assoc = {};

    while (len1--) {
        assoc[array1[len1]] = null;
    }

    while (len2--) {
        let itm = array2[len2];

        if (assoc[itm] === undefined) { // Eliminate the indexOf call
            array3.push(itm);
            assoc[itm] = null;
        }
    }

    return array3;
}

L(a1,a2);

// https://stackoverflow.com/a/39336712/860099
function M(arr1,arr2) {
  const comp = f => g => x => f(g(x));
  const apply = f => a => f(a);
  const flip = f => b => a => f(a) (b);
  const concat = xs => y => xs.concat(y);
  const afrom = apply(Array.from);
  const createSet = xs => new Set(xs);
  const filter = f => xs => xs.filter(apply(f));

  const dedupe = comp(afrom) (createSet);

  const union = xs => ys => {
    const zs = createSet(xs);  
    return concat(xs) (
      filter(x => zs.has(x)
       ? false
       : zs.add(x)
    ) (ys));
  }

  return union(dedupe(arr1)) (arr2)
}
M(a1,a2);