reduce() examples

reduce.js

// reduce()
// ========

// The way everyone starts out learning reduce() is by calculating
// the sum of an array of numbers:

const a = [1, 5, 16, 2, 7, 5, 16, 5];

// If we were going to calculate the sum of an empty array, then
// we'd want the answer to be 0. So that's what we use for the
// initial value:

//     a.reduce(..., 0);

// Illustrate intermediate values by placing a log statement in
// the callback:

let sum = a.reduce((intermediate, element) => {
  console.log(`callback called on (${intermediate}, ${element})`);
  return intermediate + element;
}, 0);
console.log(sum)

// callback called on (0, 1)
// callback called on (1, 5)
// callback called on (6, 16)
// callback called on (22, 2)
// callback called on (24, 7)
// callback called on (31, 5)
// callback called on (36, 16)
// callback called on (52, 5)
// 57

// Without the log statement, the short way of writing it is:

a.reduce((intermediate, element) => intermediate + element, 0);

// Here's an example for returning an array with duplicates
// removed. For each element, the intermediate result is all
// of the unique elements from earlier in the array, with the
// element maybe pushed in if it doesn't already exist in
// there:

a.reduce((intermediate, element) => {
  if (!intermediate.includes(element))
    intermediate.push(element);
  return intermediate;
}, []);

// [ 1, 5, 16, 2, 7 ]

// The simpler way to deduplicate array elements is to create
// a Set from the array and then spread it into an array
// again:

[...new Set(a)]

// [ 1, 5, 16, 2, 7 ]