Question about array_to_string()'s behavior and the behavior mightbe changed in the future or not

Hi, 

I have a question about array_to_string(). 
I think array_to_string() concatenates always array elements from the "beginning" of the array elements and this rule
willnot be changed on the future minor releases. 
 
Is my understanding correct? 

For example, I understand that array_to_string() "never" output like following result now and in the future, is my
understandingcorrect? 
 

[Example] -> array_to_string(ARRAY[1, 2, 3, NULL, 5], ',', '*')
[Result]  -> 2,1,3,*,5

Currently I use PostgreSQL 9.5. 

Regards,

--
Yusuke, Egashira