深入探究JavaScript中for循环的效率问题及相关优化

var i;
for(i = 0; i < someArray.length; i++) {
 var someThing = someArray[i];
 doSomeWorkOn(someThing);
}

var i,
 j;
for(i = 0; i < someArray.length; i++) {
 var someThing = someArray[i];
 for(j = 0; j < someThing.stuff.length; j++) {
   doSomeWorkOn(someThing.stuff[j]);
 }
}

_.each(someArray, function(someThing) {
 doSomeWorkOn(someThing);
})

_.each(someArray, doSomeWorkOn);

var i,
 result = [];
for(i = 0; i < someArray.length; i++) {
 var someThing = someArray[i];
 // 打到这，我已经手疼了
 if(someThing.isAwesome === true) {
   result.push(someArray[i]);
 }
}

var result = _.filter(someArray, function(someThing) {
 return someThing.isAwesome === true;
})

var result = _.map(someArray, function(someThing) {
 return trasformTheThing(someThing);
})

var grandTotal = 0,
 somePercentage = 1.07,
 severalNumbers = [33, 54, 42],
 i; // don't forget to hoist those indices;
for(i = 0; i < severalNumbers.length; i++) {
 var aNumber = severalNumbers[i];
 grandTotal += aNumber * somePercentage;
}

var somePercentage = 1.07,
 severalNumbers = [33, 54, 42],
 grandTotal;
grandTotal = _.reduce(severalNumbers, function(runningTotal, aNumber) {
 return runningTotal + (aNumber * somePercentage);
}, 0)

[3,4,5,3,3].forEach(function(obj){
  console.log(obj);
});

[1,2,3,4,5].filter(function(obj){
  return obj < 3
});

[9,8,5,2,3,4,5].map(function(obj){
  return obj + 2;
});

[1,2,3,4,5].reduce(function(pre, cur, idx, arr) {
  console.log(idx);  //4 个循环: 2-5
  return pre + cur;
});  //15

//sort方法同样很有用
[9,8,5,2,3,4,5].sort(function(obj1, obj2){
  return obj1 - obj2;
});

for (var idx = 0, len = testArray.length; idx < len; idx++) {
 //do sth.
}

for (var idx = 0, len = testArray.length; idx < len; idx++) {
 //do sth.
}

<h4 id="browser"></h4>
<table id="results" class="table"></table>

function () {

 //准备测试数据, 有200万条数据的大数组
 var testArray = []
  , testObject = {}
  , idx
  , len = 2000000
  , tmp = 0
  , $results = $("#results")
  , $browser = $("#browser")
  ;

 $browser.html(navigator.userAgent);
 $results.html('');

 for (var i = 0; i < len; i++) {
  var number = Math.random(); //若希望加快运算速度可使用取整：Math.random() * 10 | 0
  testArray.push(number);
  testObject[i] = number;
 }

 $results.append('<tr><th>测试代码</th><th>计算结果</th><th>所需时间,毫秒</th></tr>');

 //测试函数
 var test = function(testFunc) {
  var startTime
   , endTime
   , result
   ;

  startTime = new Date();
  tmp = 0;
  testFunc();
  endTime  = new Date();

  //计算测试用例(Test Case)运行所需要的时间
  result = endTime - startTime;
  $results.append('<tr><td><pre>{0}</pre></td><td>{1}</td><td>{2}</td></tr>'.format(testFunc.toString(), tmp | 0, result));
 };


 test(function() {
  //测试for in 的效率
  for (idx in testArray) {
   tmp += testArray[idx]; //经测试，idx是string类型，可能是慢的原因之一
  }
 });

 test(function() {
  //测试for loop循环的效率
  for (idx = 0, len = testArray.length; idx < len; idx++) {
   tmp += testArray[idx];
  }
 });

 test(function() {
  //测试forEach的效率
  testArray.forEach(function(data) {
   tmp += data;
  });
 });

 test(function() {
  //测试不缓存Array.length时效率
  for (idx = 0; idx < testArray.length; idx++) {
   tmp += testArray[idx];
  }
 });

 test(function() {
  //测试使用{} (Object) 存健值对时，使用for in的效率如何
  for (idx in testObject) {
   tmp += testObject[idx];
  }
 });
 
 test(function() {
  //测试从{} Object查值时的效率如何（这里的健key值事先己知）
  for (idx = 0, len = testArray.length; idx < len; idx++) {
   tmp += testObject[idx];
  }
 });

}