What is the sorting method of es6 array

The sorting method of es6 array is "sort()". The sort() method is used to sort the elements of the array. The sorting order can be alphabetical or numerical, and in ascending or descending order. The default is alphabetical ascending order; this method has an optional parameter, which must be a function, and the syntax is "array. sort(callback(a,b))".

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

The sort() method is used to sort the elements of an array.

• The sort order can be alphabetical or numerical, and in ascending or descending order.

• The default sort order is ascending alphabetically.

Among them, the sort() method has an optional parameter. However, this parameter must be a function. When calling the sort() method of an array, if no parameters are passed, the elements in the array will be sorted in alphabetical order (character encoding order). If you want to sort according to other criteria, you need to pass a parameter and it is a function. This function Compares two values ​​and returns a number describing the relative order of the two values.

Syntax:

array.sort(callback(a,b))

#Return value: Array type, which is a reference to the array. Please note that the array is sorted on the original array, no copy is made.

Example:

 //sort的基本使用
  let arr = [8, 1, 4, 3, 7, 9]
  let Arr = [21, 55, 29, 105, 45]
  console.log(arr.sort()) //[1, 3, 4, 7, 8, 9]
  console.log(Arr.sort()) // [105, 21, 29, 45, 55]

It can be seen from the above code that the sort() method can only correctly sort arrays within 0-9. Although the return value is given for array items with more than 100 digits, they are not the sorted results. This is because sort() performs internal sorting based on ASCLL codes, not based on numerical values. So this method cannot even perform formal sorting on numbers above two digits. How is it different from salted fish?

Here comes the key point: sort() can receive a callback (a, b) that carries two formal parameters, that is, a and b are two elements that are about to be compared in size, and there must be a return value.

• When the return value of callback is a positive number, then b will be arranged before a;

• When the return value of callback is a negative number , then a will be arranged before b;

• When the return value of callback is 0, then the positions of a and b remain unchanged;

• Every time sort is executed, the positions of the two parameters a and b in the original array will be exchanged based on the return value;

You will be confused after reading the above description, you must Will ask where is the return value? Who is the actual parameter of parameter a b? Once you understand the following code, these are all child’s play!

 //sort 内部写法
  let Arr = [56, 21, 29, 105, 45]
  Arr.sort(function(a, b) { //callback
      if (a > b) { // a b 分别是Arr中的 56 21
          return 1  //返回正数 ，b排列在a之前
      } else {
          return -1 //返回负数 ，a排列在b之前
      }
  })
  console.log(Arr) //[21, 29, 45, 55, 105]

Execution logic:

It should be noted that the two parameters received by callback(a, b) are a = > current item, b The next item of the current item, if the positions of the current item and the next item remain unchanged, b is the index of the next item -1; the condition for judging the end of the traversal is that the b parameter will end if it cannot obtain a value. For example, the third round in the above code When executing the second time, the index of the current item is 3, then b is the next item, that is, 4. The 4th item cannot be obtained in the array, and the conditions for continuing the traversal are not met, so the traversal ends!

Let’s talk about return values: The return values ​​1 and -1 written in the above code are just symbolic representations of 1 being a positive number and -1 being a negative number. No matter what return value you write in the code, sort will only judge you internally. Whether the return value is a positive number or a negative number, it is feasible to return 100 even if the equation is true or -10000 if it is not true.

Explanation of abbreviation:

//简写 最终版  
  let Arr = [56, 21, 88, 10, 5, 77]
  Arr.sort((a, b) => a - b) //箭头函数不加大括号指向这个函数的返回值，可以不写return关键字
  console.log(Arr) //[5, 10, 21, 56, 77, 88]

As can be seen from the above figure, the internal processing method of the callback function is a - b, instead of comparing two numbers. . This is because the step of comparing two numbers is done by sort. You only need to specify the return value. Mathematically, it happens that large numbers - decimals = positive numbers, decimals - large numbers = negative numbers

Example If 56 - 21 = 35 is a positive number, the return value is a positive number, and the positive number represents changing the position;

21 - 88 = 35 is a negative number, the return value is a negative number, and the negative number represents changing the position;

If in mathematics, large number - small number ≠ positive number, small number - large number ≠ negative number, it cannot be abbreviated like this. So it should be clear that sort internally compares each other rather than subtracts each other;

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