r - Join data tables including missing records in both of them

I want to join data tables:

> (d1 <- data.table(k=1:3,v=letters[1:3],key="k"))
   k v
1: 1 a
2: 2 b
3: 3 c
> (d2 <- data.table(k=2:4,v=LETTERS[2:4],key="k"))
   k v
1: 2 B
2: 3 C
3: 4 D

> d1[d2]
   k  v i.v
1: 2  b   B
2: 3  c   C
3: 4 NA   D
> d2[d1]
   k  v i.v
1: 1 NA   a
2: 2  B   b
3: 3  C   c

What I want is the "union" of the last two joins:

   k  v i.v
1: 1 NA   a
2: 2  B   b
3: 3  C   c
4: 4  D  NA

Thanks!

Answer

You can use merge:

merge(d2,d1,all=TRUE)