Question

ammonia is an base but does not contain hydroxly group

Answer 1

Ammonia (NH3) is a weak base, which does not have hydroxyl group in its formula but it gives OH- ions when it reacts with water.

Answer 2

Acid,Base & Salts

Ammonia(NH3) is neither metallic oxide nor a hydroxide, but still it behaves like a base

The definition of base tells that,

Defination (1)
The chemical substances which gives OH` ions with the contact of water are called base. (Arrhenius's definition of base or modern defination of base)

Defination (2)
And the chemical substances that gives salt and water with the contact of acids are called a base. (Old defination of base)

Now when ammonia reacts with acid -
NH3 + HCl ------->NH4Cl
which is a salt but there is no H2O molecules present.

And when NH3 reacts with water -
NH3 + H2O ---> NH4OH
NH4OH -----> NH4 + OH
Due to the presence of OH ions in the diluted solution of ammonia, ammonia is considered as a base.

That's it
Hope it helped (･ิω･ิ)