Source

File: wp-includes/rest-api/class-wp-rest-request.php

public function get_params() {
		$order = $this->get_parameter_order();
		$order = array_reverse( $order, true );

		$params = array();
		foreach ( $order as $type ) {
			// array_merge() / the "+" operator will mess up
			// numeric keys, so instead do a manual foreach.
			foreach ( (array) $this->params[ $type ] as $key => $value ) {
				$params[ $key ] = $value;
			}
		}

		return $params;
	}