Analyze the use and principle of sort method in JS

This article mainly analyzes the use and principle of the sort method in JS. Friends who need it can refer to it. I hope it can help everyone. Let’s take a look with the editor below.

Native JS provides two methods for reordering: reverse() and sort(). There is nothing to say about reverse(), which just directly reverses the array, such as the following example:

    var arr = [1, 2, 3, 4, 5, -1, -10, 9, 0];
    arr.reverse();
    alert(arr);
    //0, 9, -10, -1, 5, 4, 3, 2, 1

The running result is the reverse order of the array, there is nothing to say.

Let’s talk about the sort() method in detail:

The sort method can be called directly without passing in any parameters. You can also pass in a comparison function as a parameter. We will talk about the comparison function later. To explain, let’s first talk about when parameters are not passed.

When no parameters are passed in, the sort() method will call the default sorting method, that is, first call the toString() transformation method of each array item, and then sort the characters according to the string Unicode encoding order. Strings are sorted, such as the following example:

    var arr = [1, 2, 3, 15, 22, 33];
    arr.sort();
    alert(arr);
    //1, 15, 2, 22, 3, 33

The output result may not be the same as imagined, but it is sorted according to the order of these numbers in the Unicode character set.

So how can we let him sort the array according to our ideas? That's right, you just pass in a function as a parameter to specify the sorting method. (Note that the parameter must be a function, not anything else)

This function can be written as follows:

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

Let’s explain this function. If value2 is greater than value1, then 1 will be returned. , that is, true, then the exchange will be performed, so that the larger value2 will be placed in the front, and the smaller value1 will be placed in the back. In this way, the overall arrangement will be sorted from largest to smallest. The following is an example of practical application:

    var arr = [1, 2, 3, 15, 22, 33, 44, 55, 0, -1, 22, 55];
    function cmp (value1, value2) {
        if (value1 < value2) {
            return 1;
        }
        else if (value1 > value2) {
            return -1;
        } else {
            return 0;
        }
    }
    arr.sort(cmp);
    alert(arr);

Running results:

There is also a simplified way to write the cmp function:

    function cmp (a, b) {
        return b - a;
    }

This function has the same effect as the above function. When b is relatively large, it will return a number greater than or equal to 1, that is, true, so that the exchange will be performed. The overall result is also a descending sort. You can Write the code yourself and try it.

So, now that the numbers in the array have been sorted, how to sort based on a certain attribute value in the array object?

A feature of JS functions that we need to use here is to use functions as return values. You can nest a layer of functions to receive object attribute names, indicating which attribute to sort according to:

<script type="text/javascript">
    var arr = [
        { name: &#39;guo&#39;, age: 20},
        { name: &#39;yu&#39;, age: 19},
        { name: &#39;liu&#39;, age: 15}
    ];

    function cmp (property) {
        return function (a, b){
            var value1 = a[property];
            var value2 = b[property];

            return value1 - value2;
        }
    }
    arr.sort(cmp(&#39;age&#39;));
    for (var i=0; i<3; i++) {
        alert(arr[i].name);
    }
    //liu, yu, guo
</script>

Some people may not understand the current cmp function, so you can debug this code to see what is going on:

As you can see, the cmp function The property in only serves as an identifier, but in fact the data in arr is passed to the internal function, and then the parameters of the internal function are used to obtain the values ​​that need to be compared based on the property.

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