reduce() is another important method of an array.

reduce() executes a callback function on all the items of the array and allows to progressively compute a result. If initialValue is specified, accumulator in the first iteration will equal to that value.

a.reduce((accumulator, currentValue, currentIndex, array) => {
  //...
}, initialValue)

Example:

[1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) => {
  return accumulator * currentValue
}, 1)

// iteration 1: 1 * 1 => return 1
// iteration 2: 1 * 2 => return 2
// iteration 3: 2 * 3 => return 6
// iteration 4: 6 * 4 => return 24

// return value is 24