It is a high level scripting/programming language built into all modern browsers, enabling dynamic functionality to be added to static web pages, and enables the creation of single page web applications seen in everyday use from Google’s GMail application to Twitter’s website.
The first of these is the client’s device which can be a laptop, pc or mobile device running a modern browser such as Chrome, Firefox or Edge.
The API is a specification of the functionality that the application provides, it is intended to enable software components to communicate with each other. The concept behind API’s is that they should abstract away the more complex functionality of the application’s code and provide a simple, easy to use interface for other applications/programs to be able to use. When two systems connect together via an API we say that they are integrated. In an integration, there are two distinct sides: the client and the server, the server is the side that runs the program that provides the API, and the client is the side that uses/consumes the API.
OpenAPI is one of the more popular API specifications, it was previously known as Swagger, it is used to define the API endpoints as well as the format of the response messages which are defined in JSON, a machine-readable data format.
"name": "Cassio Zen",
"name": "Dan Abramov",
"name": "Pete Hunt",
Once the application has received the json data response from the API, it uses this to render a list of the contacts in the browser.