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Marouane El-anbri
Marouane El-anbri

Reputation: 51

Confusing point about Array sort method in javascript

i was wondering how javascript Array sort method works, when a custom sorting function is specified like the one below :

arr = [1, 5, 122, 12];
arr.sort(function (a,b){return a-b;});

if it only accepts two arguments a,b (1,5) in "arr", i know that javascript allow more arguments than those specified in the function. but how sort function is acting, does it compares 1,5 then 122,12, store result in a place then redo the comparison.

Upvotes: 2

Views: 299

Answers (2)

Álvaro González
Álvaro González

Reputation: 146660

when a custom sorting function is specified

This is not correct. You provide a function to compare items:

arr.sort()
arr.sort(compareFunction)

compareFunction

Specifies a function that defines the sort order. If omitted, the array is sorted according to each character's Unicode code point value, according to the string conversion of each element.

The sort algorithm itself is hard-coded in native implementation and cannot be changed. Implementations may even choose to pick different algorithms depending on data types or array length.

The rationale is that you may have different needs:

  • You may want to use binary collation and make A different from a or you may want to use Spanish collation and make a identical to á.

  • You may want to sort custom objects:

    [
    {
        city: "Madrid",
        population: 4000000
    },
    {
        city: "Cairo",
        pages: 15000000
    }
]

  • Or you may want to sort fruits and make pear come before apple :)

However, the sort algorithm itself (quicksort, etc.) is an implementation detail that normally doesn't matter for your business logic.

Upvotes: 2

rpadovani
rpadovani

Reputation: 7360

It is well explained in the MDN documentation:

  • If compareFunction(a, b) is less than 0, sort a to a lower index than b, i.e. a comes first.
  • If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements.
  • If compareFunction(a, b) is greater than 0, sort b to a lower index than a.

So there is nothing to do with having more than two arguments.

If you are asking in which order it compares them, it is up to implementation. You can find it doing a console.log in the function:

arr = [1, 5, 122, 12];
arr.sort(function (a,b){console.log('Comparing a:', a, 'and b:', b); return a-b;});

Upvotes: 0

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