How do we quantify the strength of ionic bonding?
As you know, bonding in sodium chloride is non-molecular. The lattice is held together by electrostatic interaction.
Consider the sodium ion,
Now, of course there is also an electrostatic force of interaction with every other SODIUM ion in the lattice. And, because these are LIKE charges, the force of interaction is REPULSIVE. But if you sum up ALL the attractive forces and repulsive forces of the electrostatic interaction, the attractive force that holds the lattice together wins. Much experimental and theoretical data support and informs this conclusion. Consider Born-Haber cycles. This is a useful site.