Question

why is ionic bond regarded as an extreme case of a polar covalent bond ?

Answer 1

In chemical bonds, atoms can either transfer or share their valence electrons. In the extreme case where one or more atoms lose electrons and other atoms gain them in order to produce a noble gas electron configuration, the bond is called an ionic bond.

Typical of ionic bonds are those in the alkali halides such as NaCl.

Answer 2

in extreme cases
..ionic bonds are formed by sharing of one or more electrons