Javascript 數組排序詳解

2019-11-20 14:00:19

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如果你接觸javascript有一段時間了，你肯定知道數組排序函數sort，sort是array原型中的一個方法，即array.prototype.sort(),sort(compareFunction),其中compareFunction是一個比較函數，下面我們看看來自Mozilla MDN 的一段描述:
If compareFunction is not supplied, elements are sorted by converting them to strings and comparing strings in lexicographic (“dictionary” or “telephone book,” not numerical) order. For example, “80″ comes before “9″ in lexicographic order, but in a numeric sort 9 comes before 80.

下面看些簡單的例子：

復制代碼代碼如下:

// Output [1, 2, 3]
console.log([3, 2, 1].sort());

// Output ["a", "b", "c"]
console.log(["c", "b", "a"].sort());

// Output [1, 2, "a", "b"]
console.log(["b", 2, "a", 1].sort());

從上例可以看出，默認是按字典中字母的順序來排序的。

幸運的是，sort接受一個自定義的比較函數，如下例：

復制代碼代碼如下:

function compareFunction(a, b) {
 if( a > b) {
  return -1;
 }else if(a < b) {
  return 1;
 }else {
  return 0;
 }
}
//Outputs ["zuojj", "Benjamin", "1"] 
console.log(["Benjamin", "1", "zuojj"].sort(compareFunction));

排序完我們又有個疑問，如何控制升序和降序呢？

復制代碼代碼如下:

function compareFunction(flag) {
 flag = flag ? flag : "asc";
 return function(a, b) {
  if( a > b) {
   return flag === "desc" ? -1 : 1;
  }else if(a < b) {
   return flag === "desc" ? 1 : -1;
  }else {
   return 0;
  }
 };
}
//Outputs ["1", "Benjamin", "zuojj"] 
console.log(["Benjamin", "1", "zuojj"].sort(compareFunction()));
//Outputs ["zuojj", "Benjamin", "1"] 
console.log(["Benjamin", "1", "zuojj"].sort(compareFunction("desc")));

comparFunction的排序規則是這樣的：
1.If it returns a negative number, a will be sorted to a lower index in the array.
2.If it returns a positive number, a will be sorted to a higher index.
3.And if it returns 0 no sorting is necessary.

下面我們來看看摘自Mozilla MDN上的一段話：
The behavior of the sort method changed between JavaScript 1.1 and JavaScript 1.2.為了解釋這段描述，我們來看個例子：

In JavaScript 1.1, on some platforms, the sort method does not work. This method works on all platforms for JavaScript 1.2.

In JavaScript 1.2, this method no longer converts undefined elements to null; instead it sorts them to the high end of the array.詳情請戳這里。

復制代碼代碼如下:

var arr = [];
arr[0] = "Ant";
arr[5] = "Zebra";
//Outputs ["Ant", 5: "Zebra"] 
console.log(arr);
//Outputs 6
console.log(arr.length);
//Outputs "Ant*****Zebra"
console.log(arr.join("*"));
//排序
var sortArr = arr.sort();
//Outputs ["Ant", "Zebra"] 
console.log(sortArr);
//Outputs 6
console.log(sortArr.length);
//Outputs "Ant*Zebra****"
console.log(sortArr.join("*"));