Question

当分配给函数中声明的变量并分配给较大范围内的变量时，我对执行速度的显着差异感到有些困惑。

我写了一个小小的测量函数，我在这些事情的全局范围内，这里有一些结果：

Chrome（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

......并在上方范围内

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

7600 ops / sec用于范围分配，而 300万 ops / sec用于在上限范围内分配〜 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

在这里，我们看到 4600万，而非 850万每秒操作。

衡量功能

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

我将很快用JavaScript进行一些繁重的数字运算。所有浏览器的这种行为是否一致？我应该考虑到这一点来实现功能吗？我应该在内部将参数传递给函数以避免在较高范围内访问它们吗？

......或者我错过了什么？

Answer 1

理论上它不应该对JavaScript有任何影响。变量声明在JavaScript方面非常重要，因为语言只有function scope（与阻止范围相反）。我总是遵循JavaScript大师Douglas Crockford（Javascript：The Good Parts的作者）给出的建议。

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.

请在以下网址阅读更多详情：Variable Declarations

我还建议使用Crockford JSLint: The JavaScript Code Quality Tool他也发出警告：JSLint will hurt your feelings

Javascript范围的访问性能 - 慢25倍？

1 个答案: