Javascript范围访问性能 - 25倍慢？

Question

当分配给一个函数声明的变量，并分配给上范围内的变量时，我对执行速度的显着差异有点困惑。

我已经写了一个小措施功能，我一直对这类事情在全球范围内，这里的一些结果：

铬（控制台）

measure("assignment to var in current scope", function(){ 
    var i = 1; 
    (function(){})(i) 
}) 
15:52:49 |INFO| Measure 'assignment to var in current scope' performed 76236944 operations per second 
iterations: 10000000 duration: 141 totalcalllag: 9.83 

...和上部范围

var i = 0; 
measure("assignment to var in upper scope", function(){ 
    i = 1; 
    (function(){})(i) 
}) 
15:53:22 |INFO| Measure 'assignment to var in upper scope' performed 2935124 operations per second 
iterations: 10000000 duration: 3417 totalcalllag: 9.99 

即用于分配操作/秒范围相比3百万 ops/sec在上范围内的分配比例为〜1:25！

---

IE11（控制台）

measure("assignment to var in current scope", function(){ 
    var i = 1; 
    (function(){})(i) 
}) 
15:54:14|INFO| Measure 'assignment to var in current scope' performed 46038396 operations per second 
iterations: 10000000 duration: 247 totalcalllag: 29.790000000000002 

var i = 0; 
measure("assignment to var in upper scope", function(){ 
    i = 1; 
    (function(){})(i)}) 
8669267 
15:54:49 |INFO| Measure 'assignment to var in upper scope' performed 8669267 operations per second 
iterations: 10000000 duration: 1183 totalcalllag: 29.5 

这里我们看到而不是每秒 OPS。

---

测量功能

var measure = function (desc, f) { 

    // measure the calling code first 
    var start, stop, callLag, nothing = function() { 
    }; 
    var iterations = 0; 
    var COUNT = 1000; 
    var duration = 0; 
    while (duration < 10) { 
     COUNT *= 1000; 
     start = (new Date()).getTime(); 
     while (iterations != COUNT) { 
      ++iterations; 
      nothing(); 
     } 
     stop = (new Date()).getTime(); 
     duration = stop - start; 
    } 
    callLag = duration/COUNT; 

    // measure for total time between 0.1 & 10 seconds 
    iterations = 0; 
    COUNT = 1000; 
    duration = 0; 
    while (duration < 100) { 
     COUNT *= 100; 
     start = (new Date()).getTime(); 
     while (iterations != COUNT) { 
      ++iterations; 
      f(); 
     } 
     stop = (new Date()).getTime(); 
     duration = stop - start; 
    } 

    // calculate operations per second 
    var opsPerSec = Math.floor(1000 * (iterations/(duration - (callLag * iterations)))); 

    Log.info("Measure '" + desc + "' performed " + opsPerSec.toString() + " operations per second\n" + 
     "\titerations: " + iterations + "\tduration: " + duration + "\ttotalcalllag: " + callLag*iterations); 

    return opsPerSec; 
}; 

---

问题

我将做一些重号很快与JavaScript的运算。这种行为在所有浏览器中是否一致？我是否应该考虑实现这些功能？我是否应该将参数传递给函数以避免在较高范围内访问它们？

......还是我错过了什么？

Answer 1

理论上它不应该在JavaScript中有任何区别。变量声明在JavaScript中非常重要，因为语言只有function scope（与块范围相反）。我始终遵循JavaScript大师Douglas Crockford（Javascript：The Good Parts的作者）的建议。

JavaScript does not have block scope, so defining variables in blocks can confuse programmers who are experienced with other C family languages. Define all variables at the top of the function.

请阅读更多细节：Variable Declarations

我也建议使用Crockford的JSLint: The JavaScript Code Quality Tool他还提出警告：JSLint will hurt your feelings