plugin.php

<?php
/**
 * Plugin Name: WP REST API filter parameter
 * Description: This plugin adds a "filter" query parameter to API post collections to filter returned results based on public WP_Query parameters, adding back the "filter" parameter that was removed from the API when it was merged into WordPress core.
 * Author: WP REST API Team
 * Author URI: http://v2.wp-api.org
 * Version: 0.1
 * License: GPL2+
 **/

add_action( 'rest_api_init', 'rest_api_filter_add_filters' );

 /**
  * Add the necessary filter to each post type
  **/
function rest_api_filter_add_filters() {
	foreach ( get_post_types( array( 'show_in_rest' => true ), 'objects' ) as $post_type ) {
		add_filter( 'rest_' . $post_type->name . '_query', 'rest_api_filter_add_filter_param', 10, 2 );
	}
}

/**
 * Add the filter parameter
 *
 * @param  array           $args    The query arguments.
 * @param  WP_REST_Request $request Full details about the request.
 * @return array $args.
 **/
function rest_api_filter_add_filter_param( $args, $request ) {
	// Bail out if no filter parameter is set.
	if ( empty( $request['filter'] ) || ! is_array( $request['filter'] ) ) {
		return $args;
	}

	$filter = $request['filter'];

	if ( isset( $filter['posts_per_page'] ) && ( (int) $filter['posts_per_page'] >= 1 && (int) $filter['posts_per_page'] <= 100 ) ) {
		$args['posts_per_page'] = $filter['posts_per_page'];
	}

	global $wp;
	$vars = apply_filters( 'rest_query_vars', $wp->public_query_vars );

	// Allow valid meta query vars.
	$vars = array_unique( array_merge( $vars, array( 'meta_query', 'meta_key', 'meta_value', 'meta_compare' ) ) );

	foreach ( $vars as $var ) {
		if ( isset( $filter[ $var ] ) ) {
			$args[ $var ] = $filter[ $var ];
		}
	}
	return $args;
}