The trick consist in provide an empty Array as initialValue Parameter

Copyarr.reduce(callback, [initialValue])

initialValue [Optional] Value to use as the first argument to the first call of the callback. If no initial value is supplied, the first element in the array will be used.

So the code would look like this:

Copyfunction minMax(items) {
    return items.reduce((acc, val) => {
        acc[0] = ( acc[0] === undefined || val < acc[0] ) ? val : acc[0]
        acc[1] = ( acc[1] === undefined || val > acc[1] ) ? val : acc[1]
        return acc;
    }, []);
}

Based on your code

var a = [10, 21, 13, 56];

function add(a, b) { return a + b }
function foo(a, b) { return a.concat(b) }

Array.prototype.reduce2 = function(fn, start){
  var result = start !== undefined ? start : this[0];
  for (var i = 0; i < this.length; i++) {
    result = fn(result, this[i]);
  }
  return result;
};
console.log(a.reduce(add), a.reduce2(add))         // 100 100
console.log(a.reduce(add, 10), a.reduce2(add, 10)) // 110 110
console.log(a.reduce(foo, ''), a.reduce2(foo, ''));
console.log(a.reduce(foo, 'X'), a.reduce2(foo, 'X'));