Sorting object with null values underscore sortBy
flapdoodle!

if (o.activityOrder === null)
        o.activityOrder = 99999999999;


Dude, don't do this. Really. 



Sorting idiom


Javascript supports sorting pretty much as you'll see in other languages.
Collections have sorting.
Sort functions can take a function delegate for customized sorting.
Typically, compare the desired values and return an integer that means less-than, equal-to, greater-than. Typically -1, 0, 1 respectively. 
handling null does not require converting it to a valid value in your set domain. 
Read the documentation!