使用reduce()进行计数-值从何而来？

const arr = [1, 2, 3, 4, 5];
const count = arr.reduce((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(count); // 输出：15

const arr = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 30 },
  { name: 'Charlie', age: 35 }
];
const count = arr.reduce((accumulator, currentValue) => accumulator + currentValue.age, 0);
console.log(count); // 输出：90

const arr = [1, 2, 3, 4, 5];
const count = arr.reduce((accumulator, currentValue) => {
  if (currentValue % 2 === 0) {
    return accumulator + 1;
  } else {
    return accumulator;
  }
}, 0);
console.log(count); // 输出：2