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What is an API?
An API, or application programming interface, is a defined way for software to access functionality provided by another component or system. It can spare developers from implementing every underlying detail themselves: a program uses the interface’s available operations rather than needing to know how they are built internally. MDN describes APIs as constructs made available in programming languages to help developers create complex functionality more easily (MDN: Introduction to web APIs).
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For a web API, picture a browser or app acting as a client and a remote system acting as a server. The client asks for a resource or submits data; the server processes the request and sends back a response. The API defines how software can make that exchange.
How is an API different from HTTP?
API is a broad concept: it describes an interface that software can use. HTTP is a client-server protocol commonly used by web APIs to carry requests and responses. They are related, but they are not synonyms; an API does not have to be an HTTP API.
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As MDN puts it, “HTTP is a client-server protocol: requests are sent by one entity, the user-agent (or a proxy on behalf of it)” (MDN: Overview of HTTP). In an ordinary web interaction, the client sends a request, the server handles it, and the server returns a response. HTTP is stateless between requests: each request is treated independently. Cookies can, however, help an application maintain a stateful session across requests.
What happens in an HTTP API request?
An HTTP request has a start line, optional headers, a blank line, and an optional body. The start line identifies the method and target; headers supply metadata, and a body can carry data the client is submitting. Here is the general sequence:
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- The client chooses a method and target. The method communicates the intended operation, while the target identifies the resource. A target commonly includes a path such as
/users. - The client may add headers. Headers can describe the request, such as the format the client can accept or the content type of submitted data.
- The client may include a body. A body can carry data, for example information submitted by a form.
- The server returns a response. The response includes a status code and headers, and may include a body with returned data or another representation.
MDN’s HTTP messages documentation illustrates a form submission with POST /users HTTP/1.1, content-type and content-length headers, and form data in the body. It is a documentation example, not a report of a live API call (MDN: HTTP messages).
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HTTP methods indicate what the client intends to do with a resource. Their behavior differs, so do not assume that all methods are interchangeable.
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- GET requests a representation of a resource and should be used to retrieve data, not to make a change.
- POST commonly sends data to a server, such as information submitted by a form.
Safety, idempotence, and cacheability are separate properties used to describe method behavior; they are not alternate names for the method itself. MDN’s method reference explains these distinctions and the semantics of individual methods (MDN: HTTP request methods).
How do you read an HTTP response?
The status code gives the client an outcome signal. Response headers provide metadata, and the response body may contain the requested data or other content. For example, when inspecting a call, read the status alongside the response body: the status describes the result, while the body shows what content, if any, the server returned.
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Where can you inspect web API calls?
Use the browser’s Network tab
When a webpage makes network requests, its developer tools can show the HTTP messages. Open the browser’s developer tools and select the Network tab, then inspect a request’s method, URL or path, headers, status code, and response body when present. This makes the request-and-response structure visible in a real browser interaction.
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Use curl from the command line
curl is a command-line tool that can send HTTP requests and display responses. It is useful for inspecting a server exchange outside the browser, though a particular API may require authentication or other request details before it returns useful data.
For JavaScript in the browser, MDN identifies Fetch as the commonly used HTTP-based API for making requests. The exact call and response handling depend on the application and the web API involved (MDN: Overview of HTTP).
What can make a simple API call more complex?
The basic exchange—client request, server response—is only the starting point. Authentication may be required before a server allows access. Cookies can carry session information, and caching can affect whether a response is reused. Browser security rules for cross-origin requests can also determine whether a webpage is allowed to read a response from a different origin. These details depend on the particular API and application, so inspect its documentation and the actual request rather than assuming every endpoint behaves alike.
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