register_rest_route() │ WP 4.4.0

Registers a REST API route.

Note: Do not use before the rest_api_init hook.

No Hooks.

Return

true|false. True on success, false on error.

Usage

register_rest_route( $namespace, $route, $args, $override );
$namespace(string) (required)
The first URL segment after core prefix. Should be unique to your package/plugin.
$route(string) (required)
The base URL for route you are adding.
$args(array)
Either an array of options for the endpoint, or an array of arrays for multiple methods.
Default: empty array
$override(true|false)
If the route already exists, should we override it? True overrides, false merges (with newer overriding if duplicate keys exist).
Default: false

Changelog

Since 4.4.0 Introduced.
Since 5.1.0 Added a _doing_it_wrong() notice when not called on or after the rest_api_init
Since 5.5.0 Added a _doing_it_wrong() notice when the required permission_callback argument is not set.

Code of register_rest_route() WP 6.0

wp-includes/rest-api.php 
function register_rest_route( $namespace, $route, $args = array(), $override = false ) {
	if ( empty( $namespace ) ) {
		/*
		 * Non-namespaced routes are not allowed, with the exception of the main
		 * and namespace indexes. If you really need to register a
		 * non-namespaced route, call `WP_REST_Server::register_route` directly.
		 */
		_doing_it_wrong( 'register_rest_route', __( 'Routes must be namespaced with plugin or theme name and version.' ), '4.4.0' );
		return false;
	} elseif ( empty( $route ) ) {
		_doing_it_wrong( 'register_rest_route', __( 'Route must be specified.' ), '4.4.0' );
		return false;
	}

	$clean_namespace = trim( $namespace, '/' );

	if ( $clean_namespace !== $namespace ) {
		_doing_it_wrong( __FUNCTION__, __( 'Namespace must not start or end with a slash.' ), '5.4.2' );
	}

	if ( ! did_action( 'rest_api_init' ) ) {
		_doing_it_wrong(
			'register_rest_route',
			sprintf(
				/* translators: %s: rest_api_init */
				__( 'REST API routes must be registered on the %s action.' ),
				'<code>rest_api_init</code>'
			),
			'5.1.0'
		);
	}

	if ( isset( $args['args'] ) ) {
		$common_args = $args['args'];
		unset( $args['args'] );
	} else {
		$common_args = array();
	}

	if ( isset( $args['callback'] ) ) {
		// Upgrade a single set to multiple.
		$args = array( $args );
	}

	$defaults = array(
		'methods'  => 'GET',
		'callback' => null,
		'args'     => array(),
	);

	foreach ( $args as $key => &$arg_group ) {
		if ( ! is_numeric( $key ) ) {
			// Route option, skip here.
			continue;
		}

		$arg_group         = array_merge( $defaults, $arg_group );
		$arg_group['args'] = array_merge( $common_args, $arg_group['args'] );

		if ( ! isset( $arg_group['permission_callback'] ) ) {
			_doing_it_wrong(
				__FUNCTION__,
				sprintf(
					/* translators: 1: The REST API route being registered, 2: The argument name, 3: The suggested function name. */
					__( 'The REST API route definition for %1$s is missing the required %2$s argument. For REST API routes that are intended to be public, use %3$s as the permission callback.' ),
					'<code>' . $clean_namespace . '/' . trim( $route, '/' ) . '</code>',
					'<code>permission_callback</code>',
					'<code>__return_true</code>'
				),
				'5.5.0'
			);
		}
	}

	$full_route = '/' . $clean_namespace . '/' . trim( $route, '/' );
	rest_get_server()->register_route( $clean_namespace, $full_route, $args, $override );
	return true;
}

REST API