At 22.1.3.19 Array.prototype.reduce ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element present in the array, in ascending order.

The callback function is called N - 1 times, where N is the number of elements in the array (when initialValue is not present).

Same error at 22.1.3.20 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element present in the array, in descending order.