关于Javascript循环体变量声明与初始化的效率问题

针对循环体变量声明与初始化的效率问题，将执行的简单测试代码如下：

function test(n) {
    console.time('Internally initialized');
    for (var i = 0; i < n; i++) {
        let a = i;
    }
    console.timeEnd('Internally initialized');

    console.time('Externally initialized');
    let a = 0;
    for (var i = 0; i < n; i++) {
        a = i;
    }
    console.timeEnd('Externally initialized');

    console.time('Externally declared');
    let b;
    for (var i = 0; i < n; i++) {
        b = i;
    }
    console.timeEnd('Externally declared');
}

Chromium 58.0.3029.110下的测试结果，没有考虑更多的测试条件、环境因素影响等等：

test(1000000000)
Script snippet #2:6 Internally initialized: 2894.2080078125ms
Script snippet #2:13 Externally initialized: 2867.099853515625ms
Script snippet #2:20 Externally declared: 2895.260986328125ms
undefined
test(100000000)
Script snippet #2:6 Internally initialized: 355.6640625ms
Script snippet #2:13 Externally initialized: 296.761962890625ms
Script snippet #2:20 Externally declared: 296.949951171875ms
undefined
test(10000000)
Script snippet #2:6 Internally initialized: 68.672119140625ms
Script snippet #2:13 Externally initialized: 43.97705078125ms
Script snippet #2:20 Externally declared: 35.217041015625ms
undefined
test(1000000)
Script snippet #2:6 Internally initialized: 12.23388671875ms
Script snippet #2:13 Externally initialized: 10.057861328125ms
Script snippet #2:20 Externally declared: 8.0849609375ms
undefined
test(100000)
Script snippet #2:6 Internally initialized: 0.94873046875ms
Script snippet #2:13 Externally initialized: 1.088134765625ms
Script snippet #2:20 Externally declared: 0.931884765625ms
undefined
test(10000)
Script snippet #2:6 Internally initialized: 0.1181640625ms
Script snippet #2:13 Externally initialized: 0.176025390625ms
Script snippet #2:20 Externally declared: 0.145751953125ms
undefined
test(1000)
Script snippet #2:6 Internally initialized: 0.044921875ms
Script snippet #2:13 Externally initialized: 0.04296875ms
Script snippet #2:20 Externally declared: 0.046875ms
undefined
test(100)
Script snippet #2:6 Internally initialized: 0.025146484375ms
Script snippet #2:13 Externally initialized: 0.033935546875ms
Script snippet #2:20 Externally declared: 0.02783203125ms
undefined
test(10)
Script snippet #2:6 Internally initialized: 0.024169921875ms
Script snippet #2:13 Externally initialized: 0.024169921875ms
Script snippet #2:20 Externally declared: 0.02197265625ms
undefined
test(1)
Script snippet #2:6 Internally initialized: 0.02490234375ms
Script snippet #2:13 Externally initialized: 0.032958984375ms
Script snippet #2:20 Externally declared: 0.030029296875ms
undefined

Firefox 54.0下的测试结果，没有考虑更多的测试条件、环境因素影响等等：

test(1000000000)
Internally initialized: 计时器开始
Internally initialized: 768.81ms
Externally initialized: 计时器开始
Externally initialized: 697.8ms
Externally declared: 计时器开始
Externally declared: 704.16ms
undefined
test(100000000)
Internally initialized: 计时器开始
Internally initialized: 131.84ms
Externally initialized: 计时器开始
Externally initialized: 84.42ms
Externally declared: 计时器开始
Externally declared: 70.98ms
undefined
test(10000000)
Internally initialized: 计时器开始
Internally initialized: 19.92ms
Externally initialized: 计时器开始
Externally initialized: 17.09ms
Externally declared: 计时器开始
Externally declared: 15.33ms
undefined
test(1000000)
Internally initialized: 计时器开始
Internally initialized: 3.46ms
Externally initialized: 计时器开始
Externally initialized: 3.06ms
Externally declared: 计时器开始
Externally declared: 2.75ms
undefined
test(100000)
Internally initialized: 计时器开始
Internally initialized: 1.49ms
Externally initialized: 计时器开始
Externally initialized: 1.11ms
Externally declared: 计时器开始
Externally declared: 2.14ms
undefined
test(10000)
Internally initialized: 计时器开始
Internally initialized: 1.03ms
Externally initialized: 计时器开始
Externally initialized: 1.11ms
Externally declared: 计时器开始
Externally declared: 1.11ms
undefined
test(1000)
Internally initialized: 计时器开始
Internally initialized: 1.25ms
Externally initialized: 计时器开始
Externally initialized: 0.84ms
Externally declared: 计时器开始
Externally declared: 0.82ms
undefined
test(100)
Internally initialized: 计时器开始
Internally initialized: 1.29ms
Externally initialized: 计时器开始
Externally initialized: 1.02ms
Externally declared: 计时器开始
Externally declared: 1.21ms
undefined
test(10)
Internally initialized: 计时器开始
Internally initialized: 1.29ms
Externally initialized: 计时器开始
Externally initialized: 1.1ms
Externally declared: 计时器开始
Externally declared: 1.64ms
undefined
test(1)
Internally initialized: 计时器开始
Internally initialized: 1.15ms
Externally initialized: 计时器开始
Externally initialized: 0.97ms
Externally declared: 计时器开始
Externally declared: 0.89ms
undefined

三种方式无明显差别。从一般性逻辑来分析，声明和初始化是两种操作，其中任何一种操作发生的次数越多耗时越多；但变量声明时就及时初始化是个好的编程习惯，未初始化的变量就直接参与运算常常会得到意想不到的结果，如(undefined) + 1 = NaN、(undefined) + 'string' = "undefinedstring"，当然很多编程语言对于未初始化的变量以默认类型值初始化，而Javascript默认值为undefined。另外，这里不探讨存储性能和垃圾回收机制。