Please help me understand Eval function query

[aaa2324]

• Please help to understand the logic of below query 
• eval count=if(isnull(count), -1,count)

[JacekF]

If count value is null set count value to -1, use the count value otherwise (i.e. if count value is not null do not change it).

[ITWhisperer]

This is the same as 

| fillnull value=-1 count

or 

| eval count=coalesce(count,-1)