Sorting a javascript array when the value of a field can be null?

Question

I have this function:

tests.sort(function (a, b) {
   var diff = a.title.localeCompare(b.title);
   return diff == 0 ? b.modifiedDate.localeCompare(a.modifiedDate) : diff;
});

I am using it to sort the tests array first by title and then by modifiedDate. The code was working but now I found it gives an error. When the modifiedDate is null and when this happens the compare fails.

How could I make it so that if the modifiedDate is null then the sort still works and places those rows after the rows with a modifiedDate that's not null?

i sometimes trash the arguments as i work down the possibilities, so you can do something like b=b.modifiedDate||0;a=a.modifiedDate||0; before the compare. — dandavis
– dandavis, Commented Mar 18, 2016 at 4:33

vol7ron · Accepted Answer · 2016-03-18 05:10:12Z

1

Thinking off the top of my head, to sort by title and then modifiedDate with nulls last:

tests.sort(function (a, b) {
   var diff = a.title.localeCompare(b.title),
       has_modifiedDate = a.modifiedDate && b.modifiedDate && true || false; 


  if (diff === 0) {
    if (has_modifiedDate) {
      return a.modifiedDate.localeCompare(b.modifiedDate)
    }
    else {
      if (! b.modifiedDate){
        if (! a.modifiedDate)
          return 0;
        else
          return -1;
      }
      else {
        if (! a.modifiedDate)
          return 1;
        else
          return -1;
      }
    }
  }
  else 
    return diff;
});

Note: this is untested and it's very late/early here. If this is incorrect, post and I'll delete or update; but way to tired to think.

Quick dataset you can try testing with; fill in with whatever data you want:

var tests = [
  {modifiedDate:'z',    title:'foo'},
  {modifiedDate:'a',    title:'foo'},
  {modifiedDate:null,   title:'foo'},
  {modifiedDate:'a',    title:'foo'},
  {modifiedDate:'null', title:'foo'},
  {modifiedDate:'z',    title:'foo'},
  {modifiedDate:'z',    title:'bar'},
  {modifiedDate:'a',    title:'bar'},
  {modifiedDate:null,   title:'bar'}
 ];

answered Mar 18, 2016 at 5:10

vol7ron

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5 Comments

Bergi Over a year ago

You could shorten it a lot (e.g. by omitting the second if (! a.modifiedDate)), but it appears to be correct

vol7ron Over a year ago

I haven't tested, but I think it's needed to push nulls to the bottom of the stack, because it's saying if they both have the same title and the next element has a modifieddate, but the current element doesn't. It looks like the if could be standalone and the else wrapper isn't needed

Will Over a year ago

Just curious: why the extra true before or? a.modifiedDate && b.modifiedDate && true || false. I haven't seen this approach before.

vol7ron Over a year ago

@will it's not necessary. Without it, the variable will hold a "truthy" value, with it it will hold true. So, if you want to be more explicit in the if statement you could use the exact comparison operator and do has_modifiedDate === true. Probably has a very very micro performance benefit too, since the variable will hold a smaller value compared to a date. But, it's not necessary; it's a convention I started adopting, when I expect my is_* and has_* variables to only have true or false values.

Bergi Over a year ago

@vol7ron I meant else { return 1; } would suffice as you already know that not both an not b are null

Will · Accepted Answer · 2016-03-18 05:02:05Z

0

Try this:

tests.sort(function(a, b) {
    var diff = a.title.localeCompare(b.title);
    return diff == 0 ? (a.modifiedDate ? a.modifiedDate.getTime() : Infinity) - (b.modifiedDate ? b.modifiedDate.getTime() : Infinity) : diff;
})

answered Mar 18, 2016 at 5:02

Will

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3 Comments

Bergi Over a year ago

You might even just do return diff || (… - …);

Bergi Over a year ago

I'd have upvoted this (nice solution!), but I think that .modifiedDate is a string (having a .localeCompare method) not a Date object, despite the name.

Will Over a year ago

@Bergi - I like the diff || (...) idea - thanks. It's cleaner.

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