Javascript .reduce() 没有按预期运行？

Question

我正在尝试创建一个函数，该函数将数组作为参数，然后将数组的每个元素与其索引相乘，然后相加并返回这些值。但是，我得到了无法解释的返回。

var sum = 0;
function crazy_sum(numbers){
 return numbers.reduce(function(previousValue,currentValue,currentIndex,array){
        sum += currentValue*currentIndex;
        return sum;
    },0);
};


console.log(crazy_sum([2])); //returns 0, as expected
console.log(crazy_sum([2,3])); //returns 3, as expected
console.log(crazy_sum([2,3,5])); //returns 16 when I would expect 13
console.log(crazy_sum([2,3,5,2])); //returns 35 when I would expect 19

为什么我没有得到预期的结果？该功能实际上在做什么？

Answer 1

你不需要sum，使用previousValue参数:

function crazy_sum(numbers){
 return numbers.reduce(function(previousValue,currentValue,currentIndex,array){
        previousValue+= currentValue*currentIndex;
        return previousValue;
    },0);
};