Edit in JSFiddle

var arr_1 = [1, 3, 5];
var arr_2 = [2, 3, 6, 8];

function merge_2_sorted_arrays(arr_1, arr_2) {
    var i = 0,
        j = 0,
        k = 0,
        merged_arr = [];

    while (i < arr_1.length && j < arr_2.length) {
        if (arr_1[i] < arr_2[j]) {
            merged_arr[k] = arr_1[i];
            i++;
        } else {
            merged_arr[k] = arr_2[j];
            j++;
        }
        k++;
    }

    while (i < arr_1.length) {
        merged_arr[k] = arr_1[i];
        i++;
        k++;
    }

    while (j < arr_2.length) {
        merged_arr[k] = arr_2[j];
        j++;
        k++;
    }

    document.getElementById('input').innerHTML = 'Input sorted arrays: [' + arr_1 + ']; [' + arr_2 + ']';
    document.getElementById('output').innerHTML = 'Merged sorted array: [' + merged_arr + ']';

    //return merged_arr;
}

merge_2_sorted_arrays(arr_1, arr_2);
<div id="input"></div>
<div id="output"></div>