Create a Vector of Integers and missing Values

It is also worth to note that if you do not need to do an in-place operation with append! actually in such cases it is much easier to do vertical concatenation:

julia> [[1, 2, 3]; repeat([missing], 2); 4; 5] # note ; that denotes vcat
7-element Array{Union{Missing, Int64},1}:
 1
 2
 3
  missing
  missing
 4
 5

julia> vcat([1,2,3], repeat([missing], 2), 4, 5) # this is the same but using a different syntax
7-element Array{Union{Missing, Int64},1}:
 1
 2
 3
  missing
  missing
 4
 5

The benefit of vcat is that it automatically does the type promotion (as opposed to append! in which case you have to correctly specify the eltype of the target container before the operation).

Note that because vcat does automatic type promotion in corner cases you might get a different eltype of the result of the operation:

julia> x = [1, 2, 3]
3-element Array{Int64,1}:
 1
 2
 3

julia> append!(x, [1.0, 2.0]) # conversion from Float64 to Int happens here
5-element Array{Int64,1}:
 1
 2
 3
 1
 2

julia> [[1, 2, 3]; [1.0, 2.0]] # promotion of Int to Float64 happens in this case
5-element Array{Float64,1}:
 1.0
 2.0
 3.0
 1.0
 2.0

See also https://docs.julialang.org/en/v1/manual/arrays/#man-array-literals.

This will work:

append!(Union{Int,Missing}[1,2,3], repeat([missing], 1000))

[1,2,3] creates just a Vector{Int} and since Julia is strongly typed the Vector{Int} cannot accept values of non-Int type. Hence, when defining a structure, that you plan to hold more data types within, you need to explicitly state it - here we have defined Vector{Union{Int,Missing}}.