Compare types in Javascript

Question

I saw in Crockford's Book Javascript: The Good Parts that he does typeof comparison like this:

return typeof a < typeof b ? -1 : 1;

I made my own tests and I think this is the "ordering" of the different types:

function < number < object or array < string < undefined

Is this how JS actually does the comparison?

Why would you need to test if a type is "less than" another? What does that even mean? Besides typeof() returns a string, so its really a lexographic comparison. — Chad
– Chad, Commented Dec 23, 2011 at 17:42
@Chad: Probably sorting values in an Array by type using Array.prototype.sort. Here's an example — user1106925
– user1106925, Commented Dec 23, 2011 at 17:56
@amnotiam Like I said its not going to sort based on some type precedence though, it will sort lexicographically on the type string. — Chad
– Chad, Commented Dec 23, 2011 at 17:58
@Chad: I guess I should have said "grouping by type via sorting by typeof" instead of mere sorting. I haven't read The Good Parts (and have no plan to) so I don't know what the context is. — user1106925
– user1106925, Commented Dec 23, 2011 at 18:03
@amnotiam I agree I think the context of this statement makes all the difference. Defining a grouping sort is the only thing I can think of for this... — Chad
– Chad, Commented Dec 23, 2011 at 18:05

Rob W · Accepted Answer · 2011-12-23 18:11:02Z

4

The typeof operator returns a string. String are compared by its numeric value.

So, the < comparison order would be:

type       charCode ("tfnosux".charCodeAt(i))   Example
boolean     98                                   true
function   102                                   Date
number     110                                   123
object     111                                   []
string     115                                   ""
undefined  117                                   undefined
xml        120                                   <x></x>

tfnosux are the first characters of the types. The charCodeAt method returns the numeric charCode of a character in JavaScript.

I have added an example of each type at the previous block. Most JavaScript developers know about the first types. The final type, xml, is less commonly known, and can be obtained by using typeof on EX4.

Demo of typeof: http://jsfiddle.net/9G9zt/1/

edited Dec 23, 2011 at 18:11

answered Dec 23, 2011 at 17:42

Rob W

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10 Comments

bobbymcr Over a year ago

Your list seems slightly off -- shouldn't O come before S?

Rob W Over a year ago

@bobbymcr I noticed when calculating the numeric charcode values. It has been corrected ;)

qwertymk Over a year ago

What's an example of something of xml data type?

Rob W Over a year ago

@qwertymk I've constructed an example. Have a look at the The updated answer.

Rob W Over a year ago

@Chad Wrong, the typeof xmlDocument is also returns object. Have a look at the updated answer for the true explanation + demo.

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Paul Butcher · Accepted Answer · 2011-12-23 17:50:28Z

2

It is not importance. typeof returns a string, and the comparison operators work for strings, by performing a "a simple lexicographic ordering on sequences of code point value values".

Basically, if one string starts with the other, then that is the greater of the two, otherwise the first character position that differs between the two is compared.

See section 11.8.5 of the spec

answered Dec 23, 2011 at 17:50

Paul Butcher

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