An API function with one parameter named x applied to an Association:

An API function with an optional parameter having a default value:

Use CloudDeploy to deploy an API function to the Wolfram Cloud:

The deployed API function can be invoked by accessing the cloud object in a web browser and appending ?x=value to its URL:

Calling an APIFunction with insufficient input data will return a Failure:

An API function that returns the result of the computation as Wolfram Language text inside a JSON container with metainformation about the computation:

Get a map of any country:

Find the latest stock price for a given symbol:

Plot a CellularAutomaton:

Deploy a BarChart API:

Deploy an API to create and redirect to a new FormFunction:

Deploy an API to get the population for any country in a given year:

Return a full webpage from a template:

A deployed API can be accessed from any HTTP client (given proper authentication if required):

APIFunction is designed to be interchangeable with FormFunction:

And vice versa:

APIFunction accepts the same options as FormFunction but ignores most of them:

The single fields of an APIFunction use Interpreter to transform the input in Wolfram Language expressions:

This is effectively the same as the following:

And the same as applying Interpreter directly:

An APIFunction is executed with $EvaluationEnvironment set to "WebAPI" and with $CloudEvaluation being True:

Compare this to the result in a desktop session:

The evaluation environment for CloudEvaluate is "WebEvaluation":

APIFunction is useful to perform computations on demand based on input parameters. For computations that do not depend on any input parameters, Delayed can be used as well:

Equivalent code that uses APIFunction:

The result of $CloudEvaluation is True for both executing an API function and using CloudEvaluate:

With the default setting IncludeDefinitionsTrue, deploying an API function includes all definitions it depends on:

Export and response formats are only applied when the API is executed on the web:

When manually building a query to an APIFunction, it is important to apply URLEncode to it:

Note that calling URLExecute with a list of rules will automatically encode the query parameters:

Multiple requests to the same APIFunction are not typically evaluated in the same Wolfram Engine, hence definitions do not persist between requests: