How to reduce an empty array with JavaScript?

To reduce an empty array with JavaScript, we should set the initial value returned by reduce.

For instance, we write

const val = [].reduce(
  (previousValue, currentValue) => previousValue + currentValue,
  0
);

to call reduce on an empty array with a callback that returns the partial sum of the array values.

We set the initial value of previousValue to 0 by calling it with the 2nd argument to avoid the reduce empty array error.