How to Add Your Own REST API Endpoint to WordPress

As mentioned in previous articles, WP has a rich set of built-in features for working with its API.
Today, we’ll look at how to create a custom endpoint to read data.
To implement a basic API in WordPress, we’ll need the action hook rest_api_init and the function register_rest_route.

Here is the complete code:

add_action('rest_api_init', function()
{
	/**
	 * Read
	 */
	register_rest_route('app/v1', 'read', [
		'methods' => WP_REST_Server::READABLE,
		'permission_callback' => function(WP_REST_Request $Request)
		{
			//Sets
			$test_string = $Request -> get_param('test_string');

			if(empty( $test_string ))
			{
				return new WP_Error('test_string', 'Field test_string is required', [
					'status' => 400
				]);
			}

			return true;
		},
		'callback' => function(WP_REST_Request $Request)
		{
			$test_string = $Request -> get_param('test_string');
			$test_integer = $Request -> get_param('test_integer');
			$test_enum = $Request -> get_param('test_enum');
			$test_array_integer = $Request -> get_param('test_array_integer');

			
			return [
				'test_string' => $test_string,
				'test_integer' => $test_integer,
				'test_enum' => $test_enum,
				'test_array_integer' => $test_array_integer,
			];
		},
		'args' => [
			'test_string' => [
				'description' => 'Some test string',
				'type'=> 'string',
			],
			'test_integer' => [
				'description' => 'Some test integer',
				'type'=> 'integer',
				'sanitize_callback' => 'absint',
			],
			'test_enum' => [
				'description' => 'Some test enum',
				'type' => 'string',
				'enum' => ['yes', 'no'],
				'required' => true,
			],
			'test_array_integer' => [
				'description' => 'Some test array integer',
				'type' => 'array',
				'items' => [
					'type' => 'integer',
				],
				'default' => [],
			],
		]
	]);
});

As you’ve probably guessed, to work with any WP API, we must wrap our code in a hook:

add_action('rest_api_init', function()
{
	/**
	 * API code here
	 */
});

The core of our implementation is the WordPress function register_rest_route, which registers our custom endpoint. It takes four parameters:
$namespace – the first part of the route.
$route – the second part, can include regular expressions.
$args – an array of settings for the endpoint.
$override – whether to override an existing route.

Now let’s take a closer look at the third argument ($args). We use four keys:
methods – specifies the HTTP method (GET, POST, etc.).
permission_callback – a function that runs before the main callback, useful for permission checks (e.g., is the user the post author).
callback – the function that handles the logic of the endpoint.
args – an array of expected request parameters, including validation and default values.

For example, in permission_callback, we can define our validation:

'permission_callback' => function(WP_REST_Request $Request)
{
	//Sets
	$test_string = $Request -> get_param('test_string');

	if(empty( $test_string ))
	{
		return new WP_Error('test_string', 'Field test_string is required', [
			'status' => 400
		]);
	}

	return true;
},

In this code, we check whether the variable $test_string has a value. If it’s empty, we return an error and halt execution.

In the callback block, we simply return data from the GET request, although you could implement any logic here:

'callback' => function(WP_REST_Request $Request)
{
	$test_string = $Request -> get_param('test_string');
	$test_integer = $Request -> get_param('test_integer');
	$test_enum = $Request -> get_param('test_enum');
	$test_array_integer = $Request -> get_param('test_array_integer');

	
	return [
		'test_string' => $test_string,
		'test_integer' => $test_integer,
		'test_enum' => $test_enum,
		'test_array_integer' => $test_array_integer,
	];
},

Under the “args” key, we list all the GET/POST parameters we plan to use:

'args' => [
	'test_string' => [
		'description' => 'Some test string',
		'type'=> 'string',
	],
	'test_integer' => [
		'description' => 'Some test integer',
		'type'=> 'integer',
		'sanitize_callback' => 'absint',
	],
	'test_enum' => [
		'description' => 'Some test enum',
		'type' => 'string',
		'enum' => ['yes', 'no'],
		'required' => true,
	],
	'test_array_integer' => [
		'description' => 'Some test array integer',
		'type' => 'array',
		'items' => [
			'type' => 'integer',
		],
		'default' => [],
	],
]

Interestingly, you can still access any request variable via:

$Request -> get_param('test_xxx');

But if it wasn’t listed in “args”, it won’t be filtered or validated.

As you can see, creating a custom endpoint in WordPress is quite straightforward, especially if you refer to the official documentation or read WP core code.
Good luck!