I don't see the code but to guess a bit -

[].reduce(f, acc)

acc is the initial value to use, for example when summing you start with 0:

const f = (a, b) => a + b
let acc = 0  // <-- second argument of reduce
for (const x of xs) {
    acc = f(acc, x)
}

Two big reasons why one might want to supply this is if it's empty (otherwise there can be no result) or if the accumulated value has a different type/shape from the things in the array so that the first value of the array cannot be used as the initial value (which is what happens when omitting acc)