诀窍在于提供一个empty Array as initialValue参数

The trick consist in provide an empty Array as initialValue Parameter

arr.reduce(callback, [initialValue])

initialValue [可选]用作回调
第一次调用的第一个参数的值。如果没有提供初始值，将使用数组中的第一个
元素。

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.

所以代码看起来像这个：

So the code would look like this:

function 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;
    }, []);
}