ES6 spread to the rescue

spread-example.js

// get all items of all players

/* WITHOUT ES6 spread operator */
const items = [];
players.forEach((player) => {
  player.forEach((item) => {
    items.push(item);
  });
});  
  
/* WITH ES6 spread operator */
const items = players.reduce((items, player) => […items, …player.items], []);