JavaScript中Array.sort()的底层实现及应用
JavaScript中Array.sort()的底层实现及应用1. V8 引擎的 array.js js中的sort()方法用于对数组元素进行排序，具体是如何实现的？查阅资料发现，V8 引擎 sort 函数只给出了两种排序 InsertionSort 和 QuickSort，数组长度小于等于 22 的用插入排序 InsertionSort，比22大的数组则使用快速排序 QuickSort。源码中这样写道：
// In-place QuickSort algorithm.
// For short (length <= 22) arrays, insertion sort is used for efficiency.
此外，附上其他引擎的sort实现方式
Mozilla/Firefox : 归并排序（jsarray.c 源码）
Webkit ：底层实现用了 C++ 库中的 qsort() 方法（JSArray.cpp 源码）
V8的array.js源码关于sort的部分 https://github.com/v8/v8.git
代码语言：javascript复制function InnerArraySort(array, length, comparefn) {
// In-place QuickSort algorithm.
// For short (length <= 22) arrays, insertion sort is used for efficiency.
//……
var InsertionSort = function InsertionSort(a, from, to) {
for (var i = from + 1; i < to; i++) {
var element = a[i];
for (var j = i - 1; j >= from; j--) {
var tmp = a[j];
var order = comparefn(tmp, element);
if (order > 0) {
a[j + 1] = tmp;
} else {
break;
}
}
a[j + 1] = element;
}
};
var QuickSort = function QuickSort(a, from, to) {
var third_index = 0;
while (true) {
// Insertion sort is faster for short arrays.
if (to - from <= 10) {
InsertionSort(a, from, to);
return;
}
if (to - from > 1000) {
third_index = GetThirdIndex(a, from, to);
} else {
third_index = from + ((to - from) >> 1);
}
// Find a pivot as the median of first, last and middle element.
var v0 = a[from];
var v1 = a[to - 1];
var v2 = a[third_index];
var c01 = comparefn(v0, v1);
if (c01 > 0) {
// v1 < v0, so swap them.
var tmp = v0;
v0 = v1;
v1 = tmp;
} // v0 <= v1.
var c02 = comparefn(v0, v2);
if (c02 >= 0) {
// v2 <= v0 <= v1.
var tmp = v0;
v0 = v2;
v2 = v1;
v1 = tmp;
} else {
// v0 <= v1 && v0 < v2
var c12 = comparefn(v1, v2);
if (c12 > 0) {
// v0 <= v2 < v1
var tmp = v1;
v1 = v2;
v2 = tmp;
}
}
// v0 <= v1 <= v2
a[from] = v0;
a[to - 1] = v2;
var pivot = v1;
var low_end = from + 1; // Upper bound of elements lower than pivot.
var high_start = to - 1; // Lower bound of elements greater than pivot.
a[third_index] = a[low_end];
a[low_end] = pivot;
// From low_end to i are elements equal to pivot.
// From i to high_start are elements that haven't been compared yet.
partition: for (var i = low_end + 1; i < high_start; i++) {
var element = a[i];
var order = comparefn(element, pivot);
if (order < 0) {
a[i] = a[low_end];
a[low_end] = element;
low_end++;
} else if (order > 0) {
do {
high_start--;
if (high_start == i) break partition;
var top_elem = a[high_start];
order = comparefn(top_elem, pivot);
} while (order > 0);
a[i] = a[high_start];
a[high_start] = element;
if (order < 0) {
element = a[i];
a[i] = a[low_end];
a[low_end] = element;
low_end++;
}
}
}
if (to - high_start < low_end - from) {
QuickSort(a, high_start, to);
to = low_end;
} else {
QuickSort(a, from, low_end);
from = high_start;
}
}
};
//……
}2. sort方法的使用2.1 api 语法：arrayObject.sort(sortby)；参数sortby可选，用于规定排序规则，必须是函数。
注：如果调用该方法时没有使用参数，将按字母顺序对数组中的元素进行排序(按照字符编码的顺序)
如果想按照其他标准进行排序，就需要提供比较函数，该函数要比较两个值，然后返回一个用于说明这两个值的相对顺序的数字。比较函数应该具有两个参数 a 和 b，其返回值如下：
a < b，排序后的数组中 a 在 b 前，返回一个小于 0 的值
a == b，返回 0
a > b，返回一个大于 0 的值
快速记忆秘诀
csxiaoyao的个人记忆方法为：return一个值，表示是否调换这两个数的顺序，a-b –> 小到大，b-a –> 大到小
2.2 例1：无参sort()对字符串数组排序代码语言：javascript复制var arr = new Array(5);
arr = ["csxiaoyao","sunshine","studio","sunshine studio","sun"];
console.log(arr.sort().toString());输出：
代码语言：javascript复制csxiaoyao,studio,sun,sunshine,sunshine studio2.3 例2：无参sort()对Number数组排序代码语言：javascript复制var arr = new Array(5)
arr = [80,70,700,7,8];
console.log(arr.sort().toString());输出：
代码语言：javascript复制7,70,700,8,80 注意 : 结果没有按照数值大小对数字进行排序，而是按字符串排序，要实现按数值排序，必须使用排序函数。
2.4 例3：带参sort()对字符串类型的数值数组排序代码语言：javascript复制var arr = new Array(5);
arr = ["80","70","700","7","8"];
console.log(arr.sort(sortNumber).toString());
function sortNumber(a,b){
return a - b;
}输出：
代码语言：javascript复制7,8,70,80,7002.5 例4：带参sort()对简单对象List的自定义属性排序代码语言：javascript复制var objectList = new Array();
function Person(name,age){
this.name=name;
this.age=age;
}
objectList.push(new Person('csxiaoyao',20));
objectList.push(new Person('sunshine',25));
objectList.push(new Person('sunjianfeng',26));
//按年龄从小到大排序
objectList.sort(function(a,b){
return a.age-b.age;
});
for(var i=0;i console.log(objectList[i].name+" "+objectList[i].age); }输出： 代码语言：javascript复制csxiaoyao 20 sunshine 25 sunjianfeng 26