Droplet coalescence is essential from inkjet printing to aerosols in spraying. In all such processes the drop size distribution is of key importance, which is not only governed by drop formation, but also to a large part by coalescence. However, there are still a lot of debates in how and with what dynamics the initial liquid bridge growth. Here, we visualized the coalescence in the geometries of drop-drop and drop-bath using ultrafast imaging. In combination with numerical simulations, we are able to elucidate the influence of intervening gas on coalescence. We first time experimentally observed the air pocket in coalescence that could the dynamics in the early stage. The deformation, even a few hundred nanometers, on the flat surface of the bath drive the neck growth from the theoretical prediction for the inertial fluid. Viscous fluids follow well the theory. We also developed a crossover function to describe inertia-to-viscous transition.