如何从数组中找到对象的数组属性值之和？ [重复]

Question

此问题已经在这里有了答案：

How to get the average from array of objects（6个答案）

Dynamically access object property using variable（14个回答）

关闭上个月。

我有许多国家和人口。如何在所有国家提取人口？

[
  { name: 'Poland', population: 42 000 000},
  { name: 'Belarus', population: 9 500 000},
  { name: 'Moldova', population: 3 500 000},
  { name: 'Switzerland', population: 8 400 000}
]

function calculateAverageCountryPopulation(countries) {

}

数组元素的总和并不难。但是我不知道该怎么做。

var arr = [3, 2, 5, 6];

function arraySum(array) {
  var sum = 0;
  for (var i = 0; i < array.length; i++) {
    sum += array[i];
  }
  console.log(sum);
}
arraySum(arr);

Answer 1

function calculateAverageCountryPopulation(countries) {
  return countries.map(x => x.population).reduce((acc, y) => acc + y)
}

Answer 2

使用缩小和除法。

const data = [
  { name: 'Poland', population: 42000000},
  { name: 'Belarus', population: 9500000},
  { name: 'Moldova', population: 3500000},
  { name: 'Switzerland', population: 8400000}
]
const calculateAverageCountryPopulation = (countries) => countries.reduce((a,{ population: p}) => a+=p,0)/countries.length;

console.log(calculateAverageCountryPopulation(data));