計算總和的代碼

這是ES6版的

const numbers = [10, 20, 30, 40] // sums to 100
// function for adding two numbers. Easy!
const add = (a, b) =>
  a + b
// use reduce to sum our array
const sum = numbers.reduce(add)

也可以這樣寫

var numbers = [65, 44, 12, 4];

function getSum(total, num) {
    return total + num;
}
function myFunction(item) {
    document.getElementById("demo").innerHTML = numbers.reduce(getSum);
}

比較古早的ES5寫法：

var numbers = [10, 20, 30, 40] // sums to 100
var sum = 0;
for (var i = 0; i < numbers.length; i++) {
  sum += numbers[i]
}

參考文章：

Rewriting JavaScript: Sum an Array
JavaScript Array reduce() Method