  We investigate transition to the superconducting state in the ultrathin
titanium nitride films on approach to superconductor-insulator transition.
Building on the complete account of quantum contributions to conductivity, we
demonstrate that the resistance of thin superconducting films exhibits a
non-monotonic temperature behaviour due to the competition between weak
localization, electron-electron interaction, and superconducting fluctuations.
We show that superconducting fluctuations give rise to an appreciable decrease
in the resistance even at temperatures well exceeding the superconducting
transition temperature, T_c, with this decrease being dominated by the
Maki-Thompson process.The transition to a global phase-coherent superconducting
state occurs via the Berezinskii-Kosterlitz-Thouless (BKT) transition, which we
observe both by power-law behaviour in current-voltage characteristics and by
flux flow transport in the magnetic field. The ratio T_{BKT}/T_c follows the
universal Beasley-Mooij-Orlando relation. Our results call for revisiting the
past data on superconducting transition in thin disordered films.
