reduce function javascript explained code example

Example: reduce javascript

function reduce(array, func, seed) {
  let previousResult;
  if (seed === undefined) {
    previousResult = array[0];
    for (let i = 1; i < array.length; i++) {
      previousResult = func(previousResult, array[i], i, array);
    }
    console.log(previousResult);
  } else {
    previousResult = seed;
    each(array, function(e, i, a) {
      previousResult = func(previousResult, e, i, array);
    });
  }
  return previousResult;
};