If the bond between sodium and chlorine in sodium chloride were covalent, the pair of shared electrons would be attracted to the chlorine because the effective nuclear charge on the outer electrons is +7 in chlorine but is only +1 in sodium.
ɪf ðə bˈɑːnd bᵻtwˌiːn sˈoʊdiəm ænd klˈoːɹiːn ɪn sˈoʊdiəm klˈoːɹaɪd wɜː kˈoʊvələnt, ðə pˈɛɹ ʌv ʃˈɛɹd ᵻlˈɛktɹɑːnz wʊd biː ɐtɹˈæktᵻd tə ðə klˈoːɹiːn bɪkˈʌz ðɪ ɪfˈɛktɪv nˈuːklɪɹ tʃˈɑːɹdʒ ɔnðɪ ˈaʊɾɚɹ ᵻlˈɛktɹɑːnz ɪz plˈʌs sˈɛvən ɪn klˈoːɹiːn bˌʌt ɪz ˈoʊnli plˈʌs wˈʌn ɪn sˈoʊdiəm.