Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface. explain?

Since electric field inside the conductor is zero and has no tangential component on its surface, therefore, no work is done in moving a test charge within the conductor or on its surface. It means potential difference between any two points inside or on the surface is zero. Hence, electrostatic potential is constant throughout the volume of charged conductor and has same value on its surface as inside it.