Ask any IT person, what’s the stumbling block in the Web Application Development? Majority of the time, the answer is the contract between the front-end and back-end teams.This contract is API, in whatever form it is executed and whatever it conveys. It does not matter if you have JSON or a simple array. You need to establish a data exchange between the back-end and front-end teams without losses and preferably by the end of the next sprint. Note, that the back-end and front-end developers realize how important this is. It’s seemingly easy to write an API for the transfer – just go right ahead and do it, right? Not really.And what if you need to write an API for data reception? Well, that's where the argument begins. The recipient waits patiently to be told the fields of the transmitted data. On the other hand, the developer can’t always think about their deliverables. They have problems to solve while taking on new ones tasks, and thus forget about the transfer of fields. By the time we are at the end of the sprint, everyone gets together and desperately finishes the code. Still, everyone remains self-confident and stands by their opinion.So what do we do in our company to make sure that the back-end and front-end teams exchange data for API without unnecessary nervous tension? We have not yet, come up with a solution however, mastered the API contract. To do this, we use such a wonderful tool as Postman. The method is written. At first, it gets a name and can be emptied, but we usually place a template.Most importantly, it can already be tested! By the time the transmitting party understands the structure of its JSON, the receiving party will not sit idle and will be able to test its method. Now all you have to do is just mechanically insert the desired script and drink champagne!