Sorting out JavaScript Array Sorting
Consider the following JavaScript code:
var a = [30,2,1,9,15];a.sort();alert(a);
What would be output?If you’re expecting 1,2,9,15,30, you’ll be surprised to hear the actual result is 1,15,2,30,9. Don’t give up on JavaScript just yet; array sorting is incredibly powerful once you know how to use it.So what’s going on? When nothing is passed to the sort method, every value is converted to a string and sorted in lexicographic order, i.e. “15” will come before “2”. So will “10” and “19999999”.To fix the problem, we need to pass a comparison function to the sort() method. The function must take 2 parameters — we’ll name them a and b — and return:
 a value less than zero if a is less than b
 a value greater than zero if a is greater than b
 zero if a and b are equal
The simplest numeric compare function is therefore:
function compare(a, b) { return a  b;}
We can pass the compare function as as argument for the sort method or write it inline, e.g.
var a = [30,2,1,9,15];a.sort(function(a,b) { return ab; });alert(a);
The output is now a far more logical 1,2,9,15,30.One of the great things about JavaScript is that our comparison functions can sort any type of object by any property. For example, we’ll define a list of locations and home coordinates:
// location coordinatesvar locations = [ { name: "shops", x:3, y:4 }, { name: "library", x:5, y:3 }, { name: "pub", x:1, y:2 }];// home coordinatesvar home = { name: "home", x:0, y:0 };
Next, we’ll add a little Pythagoras to a function which calculates the distance between two points:
// distance between 2 coordinatesfunction distance(p1, p2) { return Math.sqrt(Math.pow(p1.xp2.x,2)+Math.pow(p1.yp2.y,2));}
We can now sort the locations by distance from home — shortest to the furthest trip:
// sort by shortest distance homelocations.sort( function(a, b) { return distance(home,a)distance(home,b); });// locations sorted: pub, shops, library
Or we could sort by furthest to shortest distance by reversing the parameters:
// sort by furthest distance homelocations.sort( function(b, a) { return distance(home,a)distance(home,b); });// locations sorted: library, shops, pub
Or we can order location names alphabetically:
locations.sort( function(a, b) { if (a.name < b.name) return 1; if (a.name > b.name) return 1; return 0; });// locations sorted: library, pub, shops
It’s easy to develop a range of reusable sorting functions which can be applied to any object containing similar property names. In that respect, JavaScript sorting is easier and more flexible than many other languages.
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