JS配列の和を求める5つの方法

unction sum(arr) {
    var len = arr.length;
    if(len == 0){
        return 0;
    } else if (len == 1){
        return arr[0];
    } else {
        return arr[0] + sum(arr.slice(1));
    }
}

function sum(arr) {
    var s = 0;
    for (var i=arr.length-1; i>=0; i--) {
        s += arr[i];
    }
    return s;
}

function sum(arr) {
    return arr.reduce(function(prev, curr, idx, arr){
        return prev + curr;
    });
}

function sum(arr) {
    var s = 0;
    arr.forEach(function(val, idx, arr) {
        s += val;
    }, 0);
  
    return s;
};

function sum(arr) {
    return eval(arr.join("+"));
};