Sorting Object by sub-object property

Question

I have an object of objects, which I'd like to sort by property... having some trouble wrapping my head around it:

sample = {
    "Elem1": { title: "Developer", age: 33 },
    "Elem2": { title: "Accountant", age: 24 },
    "Elem3": { title: "Manager", age: 53 },
    "Elem4": { title: "Intern", age: 18}
}

My expected result would be an object whose keys were now ordered Elem4, Elem2, Elem1, Elem3. Alternatively, I'd be fine with simply returning the keys in that order rather than physically sorting the object.

Is this more trouble than it's worth, or am I missing some obvious (or not-so-obvious) JavaScript-Fu that would make light work of something like this?

Thanks!

Answer 1

Properties (keys) of an object are not intrinsically ordered; you must maintain your own array of their ordering if you wish to do so.

Here is an example of how you could simplify ordering your sample object by arbitrary properties via custom sort functions:

var orderKeys = function(o, f) {
  var os=[], ks=[], i;
  for (i in o) {
    os.push([i, o[i]]);
  }
  os.sort(function(a,b){return f(a[1],b[1]);});
  for (i=0; i<os.length; i++) {
    ks.push(os[i][0]);
  }
  return ks;
};

orderKeys(sample, function(a, b) {
  return a.age - b.age;
}); // => ["Elem4", "Elem2", "Elem1", "Elem3"]

orderKeys(sample, function(a, b) {
  return a.title.localeCompare(b.title);
}); // => ["Elem2", "Elem1", "Elem4", "Elem3"]

Once the properties are ordered as you like then you can iterate them and retrieve the corresponding values in order.

Answer 2

An Object in JavaScript has no order for properties; they may come in any order when iterated over (though in practice they usually come in the order they are defined).

Use an Array, which has an explicit order, and the sort() method.

If it were an Array, you could use...

sample.sort(function(a, b) {
    return a.age - b.age;
});

jsFiddle.