JavaScript sorting using function

Question

While doing an online course I came across the following code. I am unable to understand the logic of this method. I will be grateful if someone can help me understand the code:

var v=[30,2,1,9,15,10,55,20,45,30,25,40,50,35];
var msg = document.getElementById("message")
msg.innerHTML +="<p><strong>Original Order:</strong>[" + v + "]<p>";

v.sort();
msg.innerHTML +="<p><strong>After Sorting Order(string):</strong>[" + v + "]<p>";

v.sort(function(a,b){return a-b;});
msg.innerHTML +="<p><strong>After Sorting Order(Ascending):</strong>[" + v + "]<p>";

v.sort(function(a,b){return b-a;});
msg.innerHTML +="<p><strong>After Sorting Order(Descending):</strong>[" + v + "]<p>";

I need to understand function(a,b){return a-b} - how is this function helping JavaScript sorting? Please note I am new to programming.

Look here: https://developer.mozilla.org/ru/docs/Web/JavaScript/Reference/Global_Objects/Array/sort — Nikolay Ermakov, Jan 16 '16 at 04:14

score 2 · Answer 1 · answered Jan 16 '16 at 05:02

Although others have provided many great links I wanted to share one way I think about the most basic of sort functions.

Imagine you have a very basic array:

var test = [1, 10];

The sort function accepts a function as a parameter with the signature:

function (firstVal, secondVal) { 
  return NUMBER;
}

When you apply sort to the test array

test.sort(function(firstVal, secondVal) {

  // firstVal = 1
  // secondVal = 10

  // NOTES:
  // return NEGATIVE_NUMBER if firstVal should come *before* secondVal
  // return 0 if firstVal equals secondVal
  // return POSITIVE_NUMBER if firstVal should come *after* secondVal

  // return firstVal - secondVal;  // -9; 1 will come before 10; [1, 10]
  return secondVal - firstVal; // 9; 10 will come before 1; [10, 1];
});

That function will then be ran over all the values in the array. But if you only think about 2 value I find it much easier to reason about the sort method. Javascript will handle the algorithm for you and I believe some of the links mentioned will cover how it accomplishes that.

score 0 · Answer 2 · answered Jan 16 '16 at 04:47

The JavaScript sort function takes the returned value of the function (that is the parameter of the sort function) to decide how to move the array round a is a value in the array b is a value right next to it. If a negative number is returned it moves b in front of a if a positive number is returned it moves a in front of b if 0 is returned they stay where they are. This is run over and over for every value in the array until the array no longer changes. As others have said a more detailed description can be found on the Mozilla site

JavaScript sorting using function

2 Answers2