返回值是积累的结果
// good
let sum = 0;
numbers.forEach((num) => sum += num);
sum === 15;
// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;
// Define the callback function.
function appendCurrent (previousValue, currentValue) {
return previousValue + "::" + currentValue;
}
// Create an array.
var elements = ["abc", "def", 123, 456];
// Call the reduce method, which calls the callback function
// for each array element.
var result = elements.reduce(appendCurrent);
document.write(result);
// Output:
// abc::def::123::456
