○ ~
❯ cat f
        var flatten = function(result, next_array) {
            console.log('current result', result);
            return result.concat(next_array);
        };

        [1, [2], [3, 4]].reduce(flatten);
○ ~
❯ awk  -F '\\S.*' '{l=length($1);if(l>0){if(NR==1)s=l; else s=s>l?l:s;}a[NR]=$0}END{for(i=1;i<=NR;i++){sub("^ {"s"}","",a[i]);print a[i]}}' f
        var flatten = function(result, next_array) {
  console.log('current result', result);
  return result.concat(next_array);
        };

        [1, [2], [3, 4]].reduce(flatten);