Sorting method sort of Array

The Array object in JavaScript has its own sort() method, which sorts the items in the array, but sometimes the sorting result is not satisfactory, such as

var arr = [12, 1, 2, 21, 3];
arr.sort();
alert(arr);   The result is 1,12,2,21,3

Why is that? Because JavaScript's default sorting is based on ASCII character codes, which means numbers are sorted according to their string representation.

var strArr = ['a', '}}2', 'a2', '2a', 'b', '3'];
alert(strArr.sort());

What does this result in? 2,2a,3,a,a2,b

Because the ASCII code of numbers is smaller than that of letters, so numbers are in front and letters are at the back.

How to continue to sort the array arr above in numerical order?

JavaScript provides an entry point that allows you to pass a parameter to the sort() method, namely the comparison function, to tell the sorting algorithm the relationship between values and values is greater than, less than, or equal to.

Comparison functions are functions with specific algorithms.

function compare_fn(value1, value2) {
 < value1 else if (value2) {
 return -1;
 }1 > value2) {
 return 1;
 } else {
 return 0;
 }
}

Pass the comparison function compare_fn to sort, then sort, and then output

arr.sort(compare_fn);
alert(arr);  gets 1, 2, 3, 12, 21

The definition of the return value of the sort method of the JavaScript Array object is

Negative: if the first parameter passed is less than the second parameter

Zero: the two values are equal

Positive: if the first parameter is greater than the second parameter

The comparison function above can also be abbreviated as

function compare_fn(value1, value2) {
 return value1 - value2;
}

This comparison is for ascending order

If you want to sort in descending order, just change the sign of the return value above, and take the inverse of all returns.

The comparison function for the abbreviated form is

function compare_fn(value1, value2) {
 return -(value1 - value2);
}   

that is

function compare_fn(value1, value2) {
 return value2 - value1;
}

The simple notation is: ascending in order; descending in reverse.

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