Both ${\text{NF}}_{\text{3}}$  and  ${\text{NCl}}_{\text{3}}$  are covalent but they differ in the extent of hydrolysis because

Electronegativity of fluorine is greater than chlorine

${\text{NF}}_{\text{3}}$  is more stable than ${\text{NCl}}_{\text{3}}$  and hydrolysis product of ${\text{NF}}_{\text{3}}$ , HOF do not exist

Dipolemoment of ${\text{NF}}_{\text{3}}$  is greater than that of  ${\text{NCl}}_{\text{3}}$ 

Chlorine can expand its octet by using d-orbitals