只是为了好玩:

var elmt = [0, 1, 2,3, 4, 7, 8, 9, 10, 11], l = elmt.length, i = -1, sum = 0;
for (; ++i < l; sum += elmt[i])
    ;
document.body.appendChild(document.createTextNode('The sum of all the elements is: ' + sum + ' The average of all the elements is: ' + (sum / l)));

使用reduce和ES6计算平均值(mean):

const average = list => list.reduce((prev, curr) => prev + curr) / list.length;

const list = [0, 10, 20, 30]
average(list) // 15

我只是基于Abdennour TOUMI的回答。原因如下:

1)。我同意Brad的观点，我不认为扩展我们没有创建的对象是一个好主意。

2)数组。长度在javascript中是完全可靠的，我更喜欢数组。因为a=[1,3];a[1000]=5;，现在a.length将返回1001。

function getAverage(arry){
    // check if array
    if(!(Object.prototype.toString.call(arry) === '[object Array]')){
        return 0;
    }
    var sum = 0, count = 0; 
    sum = arry.reduce(function(previousValue, currentValue, index, array) {
        if(isFinite(currentValue)){
            count++;
            return previousValue+ parseFloat(currentValue);
        }
        return previousValue;
    }, sum);
    return count ? sum / count : 0; 
};

在阅读了其他选项之后，我将尝试为未来的观众创建一个更简单的版本，详细说明现有的代码，而不是创建一个更优雅的代码。首先，您将数字声明为字符串。除了.parseInt，我们还可以做:

const numberConverter = elmt.map(Number);

map所做的就是“返回原始数组的副本”。但是我把它的值转换成数字。然后我们可以使用reduce方法(它也可以更简单，但我写的是易于阅读的版本，我也有2个平均方法)reduce方法所做的是，它有一个累加器，当它遍历数组并添加(在这种情况下)currentValue时，如果你向它添加值，它会变得越来越大。

var i;
const elmt = new Array();
elmt[0] = '0';
elmt[1] = '1';
elmt[2] = '2';
elmt[3] = '3';
elmt[4] = '4';
elmt[5] = '7';
elmt[6] = '8';
elmt[7] = '9';
elmt[8] = '10';
elmt[9] = '11';

console.log(elmt);

const numberConverter = elmt.map(Number);

const sum = numberConverter.reduce((accumulator, currentValue) => {
  return accumulator + currentValue;
}, 0);

const average = numberConverter.reduce(
  (accumulator, currentvalue, index, numArray) => {
    return accumulator + currentvalue / numArray.length;
  },
  0
);

const average2 =
  numberConverter.reduce(
    (accumulator, currentValue) => accumulator + currentValue,
    0
  ) / numberConverter.length;

for (i = 9; i < 10; i++) {
  console.log(
    `The sum of all the elements is: ${sum}. <br> The average of all the elements is: ${average2}`
  );}

将for循环计数器设置为0....你得到了元素9，然后你就完成了。其他答案都是基础数学。使用一个变量来存储你的和(需要将字符串转换为整数)，然后除以你的数组长度。

如果你需要平均值并且可以跳过计算和的要求，你可以通过调用reduce来计算平均值:

// Assumes an array with only values that can be parsed to a Float
var reducer = function(cumulativeAverage, currentValue, currentIndex) {
  // 1. multiply average by currentIndex to find cumulative sum of previous elements
  // 2. add currentValue to get cumulative sum, including current element
  // 3. divide by total number of elements, including current element (zero-based index + 1)
  return (cumulativeAverage * currentIndex + parseFloat(currentValue))/(currentIndex + 1)
}
console.log([1, 2, 3, 4, 5, 6, 7, 8, 9, 10].reduce(reducer, 0)); // => 5.5
console.log([].reduce(reducer, 0)); // => 0
console.log([0].reduce(reducer, 0)); // => 0
console.log([].reduce(reducer, 0)); // => 0
console.log([,,,].reduce(reducer, 0)); // => 0
console.log([].reduce(reducer, 0)); // => 0