Javascript範圍訪問性能 - 25倍慢？

Question

當分配給一個函數聲明的變量，並分配給上範圍內的變量時，我對執行速度的顯着差異有點困惑。

我已經寫了一個小措施功能，我一直對這類事情在全球範圍內，這裏的一些結果：

鉻（控制檯）

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 

...和上部範圍

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 

即用於分配操作/秒範圍相比3百萬 ops/sec在上範圍內的分配比例爲〜1:25！

---

IE11（控制檯）

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 

這裏我們看到而不是每秒 OPS。

---

測量功能

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; 
}; 

---

問題

我將做一些重號很快與JavaScript的運算。這種行爲在所有瀏覽器中是否一致？我是否應該考慮實現這些功能？我是否應該將參數傳遞給函數以避免在較高範圍內訪問它們？

......還是我錯過了什麼？

Answer 1

理論上它不應該在JavaScript中有任何區別。變量聲明在JavaScript中非常重要，因爲語言只有function scope（與塊範圍相反）。我始終遵循JavaScript大師Douglas Crockford（Javascript：The Good Parts的作者）的建議。

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.

請閱讀更多細節：Variable Declarations

我也建議使用Crockford的JSLint: The JavaScript Code Quality Tool他還提出警告：JSLint will hurt your feelings