JavaScript怎样实现累加

1，

function sum(arr) {

    var s = 0;

    for (var i=arr.length-1; i>=0; i--) {

        s += arr[i];

    }

    return s;

}

2，

function sum(arr) {

    return arr.reduce(function(prev, curr){

        return prev + curr;

    });

}

3，

function sum(arr) {

    var s = 0;

    arr.forEach(function(val) {

        s += val;

    });

    return s;

};

4，

function sum(arr) {

    return eval(arr.join("+"));

};