Javascript (四)：Deep Into 'Array'

Array.sort(comparator)

• no comparator Array.sort() do string comparison(ASCII)

  [1, 2, 21, 3, -4].sort();
  //=> -4, 1, 2, 21, 3

• self-define comparator

  var employees = [];
  employees[0] = {
  name: “George”,
  age: 32,
  retiredate: “March 12, 2014”
  };
  
  
  employees[1] = {
  name: “Edward”,
  age: 17,
  retiredate: “June 2, 2023”
  };
  
  
  employees[2] = {
  name: “Christine”,
  age: 58,
  retiredate: “December 20, 2036”
  };
  
  
  employees[3] = {
  name: “Sarah”,
  age: 62,
  retiredate: “April 30, 2020”
  };
  
  
  // retiredate ASC
  employees.sort(function(a, b) {
  return new Date(a.retiredate) - new Date(b.retiredate)
  });
  
  
  // age ASC
  employees.sort(function(a, b) {
  return a.age - b.age
  });
  

  类似

Array.forEach(callback)

The Array.forEach method, added in the fifth edition of the ECMA-262 language standard, take callback function and passes each array element to the function.

不同方式数组遍历、性能对比

计算一千万个数字相加求和，e1是包含一千万个数字的数组，对比结果如下：

• 通过indexed定位元素

  console.time(“[Index] locate element”);
  var total2 = 0;
  for (var i = 0; i < e1.length; i++) {
  total2 = total2 + e1[i];
  }
  console.timeEnd(“index locate element”);
  //=> [Index] locate element: 160.000ms
  

  通过下标，准确定位，性能最佳。

• 传递参数、调用算法函数

  function iterator(group) {
  var total0 = 0;
  for (var i = 0; i < group.length; i++) {
      total0 = total0 + group[i];
  }
  }
  console.time(“function call”);
  iterator(e1);
  console.timeEnd(“function call”);
  //=> function call: 180.000ms
  

  [性能损耗] 参数传递，伴随”入栈、出栈“。

• javascript native forEach

  console.time(“js native forEach”);
  var total1 = 0;
  e1.forEach(function(item) {
  total1 = total1 + item;
  });
  console.timeEnd(“js native forEach”);
  //=> js native forEach: 567.000ms

  [性能损耗] 每个元素都去调用Function

• [for in ] iterator element

  console.time(“for in locate element”);
  var total3 = 0;
  for (var g in e1) {
  total3 = total3 + g;
  }
  console.timeEnd(“for in locate element”);
  //=> for in locate element: 3054.000ms

  [性能损耗] 几乎每个元素，都去iterator数组。性能最差。

Summary

针对底层数据结构是基于数组的集合：在进行大数据遍历时，建议通过indexed定位、避免全数组遍历。 如：java中ArrayList