JavaScript null is a wily beast

js> isNaN(null);
false
js> null > -1;
true
js> null < 1;
true
js> null > 0;
false
js> null < 0;
false
js> null == 0;
false

The main tip with using non-strict equality is ToNumber conversion. When it's not obvious which result will have the comparison, just start to apply ToNumber to operands, until you rich the same types, i.e. numbers. Another case -- to use strict === equality ;)

P.S.: http://dmitrysoshnikov.com/notes/note-2-ecmascript-equality-operators/

@BrendanEich as always, the history is fascinating - thanks!

this also bothers me:

js> isNaN(null)
false
js> parseFloat(null)
NaN

it makes sense if you read what parseFloat is supposed to do but still very weird...

@millermedeiros I was confused the same way with parseInt, but I guess it made a little more sense to me that explicit conversions might behave different from implicit coercions. That's why the double-equal thing seemed particularly weird. @BrendanEich Thanks for chiming in with some context and history on this. :)