reduce.js

let myArray = ['Pizza', 'Apple', 'Banana'];
let combinedString = myArray.reduce((combinedString, item, index) => {
  if (index != 0) {
    return `${combinedString} and ${item}`;
  } else {
    return item;
  }
}, '');
console.log(combinedString); // Pizza and Apple and Banana

let myArray = [{id: 4457, name: 'Pizza'}, {id: 5678, name: 'Apple'}, {id: 2234, name: 'Banana'}];
let objectByName = myArray.reduce((objectByName, item) => {
    objectByName[item.name] = item.id;
    
    return objectByName;
}, {});
console.log(objectByName); // Object {Pizza: 4457, Apple: 5678, Banana: 2234}

let myArray = [1, 3, 5, 10];
let arrayFilled = myArray.reduce((collector, number) => {
    return collector.concat(number, 0);
}, []);
console.log(arrayFilled); // [1, 0, 3, 0, 5, 0, 10, 0]

map gives 1 to 1 return of something in place of what the array item's value is.

filter will remove items not fulfilling a requirement.

reduce kind of sort of provides both these things - and can return whatever format you want.
Just keep in mind, you always start with your 'current value'. That current value, in the examples above start as '', {} and [] respectively. You could even start with a pre-filled array. .reduce(funcToDoStuff, initialValue).

On each iteration, you must return your same/new/modified object (in the last example I called it a collector). If you don't return anything, on the next iteration, the collector var would have been undefined.