How does this compare is work? Step by step. What is mean " return 1" "return -1"? How "return" value affect for sort ?

Hi Venera,

The sort method can optionally take a compare function as an argument if you need a sort order that's different from the default sort order which is to sort based on Unicode code points. This means a character with a lower code point will come before one with a higher code point. This is how your example is sorting so you don't technically need the compare function here.

If you need something different from this then you need a compare function.

The idea here is that the sort method will pass 2 elements at a time to the compare function and the job of the compare function is to return a value to indicate what order the 2 elements should be in.

If a negative value is returned, doesn't have to be -1, then 'a' should come before 'b'

If a positive value is returned, doesn't have to be 1, then 'b' should come before 'a'

If zero is returned then the 2 elements are considered equal.

Let's say that you wanted to sort those strings based on their lengths, shortest strings first. Now you would need a compare function because the sort method does not know how to do this.

Using your array example, suppose that the sort method passes 'zo' for a and 'Ams' for b. 'zo' is the shorter string so that means that a should come before b in this case. That means that our compare function better return a negative value to indicate that. If the sort method gets back a negative value then it knows it needs to make sure that 'zo' comes before 'Ams' in the final sorted array.

The compare function would look like this:

function(a,b) {
    if (a.length < b.length) {
        return -1;
    }

    if (a.length > b.length) {
        return 1; 
    }

    return 0; }

It can be shortened down to this,

function(a, b) {
    return a.length - b.length;
}

Here's some output to show that they're sorted by string length in increasing order:

> var items = ['4' , '2' , 'zo', 'bo', '1' ,'Ams', 'B', 'a', 'A'];
undefined
> items.sort(function(a, b) { return a.length - b.length })
[ '4', '2', '1', 'B', 'a', 'A', 'zo', 'bo', 'Ams' ]
>

Here's the mdn page for the sort method if you want to know more details: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort

var a = [2, 9, 9]; 
a.indexOf(2); // 0 
a.indexOf(7); // -1

if (a.indexOf(7) === -1) {
  // element doesn't exist in array
}

-1 means the item doesn't exist in the array. That's what that means.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf

You can read more about this in the documentation.