The nitrosation of N, N-dimethylaniline takes place through the attack of electrophile

The actual nitrosation agent is the nitrosyl cation, $\mathrm{N}{\mathrm{O}}^{+}$ which is formed as follows



Aromatic tertiary amine i.e., N, N-dimethylaniline, undergo electrophilic substitution with nitrosonium ion at p-position of phenyl ring.