The reaction 2NaHCO3 -> Na2CO3 + H2O + CO2 is indeed endothermic and requires heat, as are many others that only occur on heating.
But not all, for combustion, a highly exothermic reaction, needs heat to start it when, as is usual, strong bonds in the reactants must first be broken. This first step is endothermic and so requires heat, but subsequent steps release more heat than was required to start the reaction. Reactions like these are usually self-sustaining once begun, as in flames and explosions.
In an endothermic reaction heat must be continuously supplied and the reaction is not self-sustaining. Stop heating NaHCO3 and the decomposition will slow down and stop.