var array1 = [1,2,3,4];
var array2 = [5,-6,7,8.5];

const concatArr = (arr1, arr2, sep) => arr1.map(function (num, idx) {
  return num.toString().concat(sep,(arr2[idx]).toString())
});

console.log(concatArr(array1,array2,"#")) //["1#5", "2#-6", "3#7", "4#8.5"]