for循环加速

受@wangfulin和@SyuTingSong的启发，改了下代码，
经测试，sumAll2更耗时，而在node环境下并无太大差异

    function sumAll(arr) {
        var i, sum = 0;
        for (i = 0; i < arr.length; i++) {
            sum += arr[i];
        }
        return sum;
    }

    function sumAll2(arr) {
        var len = arr.length,
            sum = 0;
        for (; len> 0;) {
            sum += arr[--len];
        }
        return sum;
    }

    var num = [],
        t2sum = 0,
        t4sum = 0,
        n = 10; //测试次数

    for (i = 0; i < 10000000; i++) {
        num[i] = parseInt(Math.random() * 100000);
    }


    function test() {
        var t1 = (new Date()).getTime();
        var x = sumAll(num);
        var t2 = (new Date()).getTime() - t1;
        t2sum += t2;

        var t3 = (new Date()).getTime();
        var y = sumAll2(num);
        var t4 = (new Date()).getTime() - t3;
        t4sum += t4;

        console.log('x:' + x);
        console.log('y:' + y);
        console.log('t2:' + t2);
        console.log('t4:' + t4);
    }

    for (i = 0; i < n; i++) {
        test();
    }
    console.log('###############################');
    console.log('t2avg:' + t2sum / n);
    console.log('t4avg:' + t4sum / n);

为神马sumAll2更耗时间

---

改了一下你的程序，方便多次运行

function sumAll(arr) {
    var i, sum = 0;
    for (i = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}

function sumAll2(arr) {
    var len = arr.length,
        sum = 0;
    for (; len > 0;) {
        sum += arr[--len];
    }
    return sum;
}

function test() {
    var num = [];
    for (var i = 0; i < 10000000; i++) {
        num[i] = parseInt(Math.random() * 100000);
    }
    var t1 = (new Date()).getTime();
    var x = sumAll(num);
    var t2 = (new Date()).getTime() - t1;

    var t3 = (new Date()).getTime();
    var y = sumAll2(num);
    var t4 = (new Date()).getTime() - t3;

    console.log('x:' + x);
    console.log('y:' + y);
    console.log('t2:' + t2);
    console.log('t4:' + t4);
}

for (var i = 0; i < 4; i++) {
    test();
}

下面是我电脑上的运行结果

x:499999293271
y:499999293271
t2:21
t4:20
x:499911919107
y:499911919107
t2:25
t4:19
x:500178188376
y:500178188376
t2:20
t4:20
x:499878909169
y:499878909169
t2:20
t4:20

说明运行耗时有波动

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我修改一下答案....看来顺序读和倒序读并没有区别.
我在nodejs下试了试, 两个时间消耗是一样的.

function sumAll3(arr) {
  var left = arr.length % 8, sum = 0;
  var i = 0, length = arr.length;
  for (; i < length;) {
    sum += arr[i + 0];
    sum += arr[i + 1];
    sum += arr[i + 2];
    sum += arr[i + 3];
    sum += arr[i + 4];
    sum += arr[i + 5];
    sum += arr[i + 6];
    sum += arr[i + 7];

    i += 8;
  }
  switch (left) {
    case 7: sum += arr[i++];
    case 6: sum += arr[i++];
    case 5: sum += arr[i++];
    case 4: sum += arr[i++];
    case 3: sum += arr[i++];
    case 2: sum += arr[i++];
    case 1: sum += arr[i++];
  }
  return sum;
}

我写了一个手动内联的版本, 表示在nodejs下面, 性能和第一个第二个没有显著差别......nodejs优化还是挺好的

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多测几次你会发现，t2有可能比t4小，也可能比t4大，len在循环内计算和循环外直接计算好对性能影响很小，所以受计算环境的影响可能会影响最终的结果，而这种结果也是不可预期的，所以没法作结论说sumAll2更耗时间或者sunAll2更不耗时间