WordPress Rest API Custom Filters (Video Tutorials Notes) - Check out the video: https://www.youtube.com/watch?v=5rSfAkLO5eo

wl-api.php

<?php
/**
 * Plugin Name: Custom API
 * Plugin URI: http://chrushingit.com
 * Description: Crushing it!
 * Version: 1.0
 * Author: Art Vandelay
 * Author URI: http://watch-learn.com
 */

function wl_products($params) {
    $price = json_decode($params->get_param('price'));

    function queryArgument($param, $key) {
        if(is_object($param)) {
            if($param->lt && $param->gt) {
                return [
                    [
                        'key' => $key,
                        'value' => [$param->gt, $param->lt],
                        'type'  => 'NUMERIC',
                        'compare' => 'BETWEEN'
                    ]
                ];
            }

            if($param->lt) {
                return [
                    [
                        'key' => $key,
                        'value' => $param->lt,
                        'type'  => 'NUMERIC',
                        'compare' => '<'
                    ]
                ];
            }

            if($param->gt) {
                return [
                    [
                        'key' => $key,
                        'value' => $param->gt,
                        'type'  => 'NUMERIC',
                        'compare' => '>'
                    ]
                ];
            }
        }


        if($param) {
            return [
                [
                    'key' => $key,
                    'value' => $param,
                    'type'  => 'NUMERIC'
                ]
            ];
        }

        return null;
    }

	$args = [
		'posts_per_page' => 99999,
        'post_type' => 'product',
        'meta_query' => queryArgument($price, 'price')
	];

    $posts = new WP_Query($args);

	$data = [];
	$i = 0;

	foreach($posts->posts as $post) {
		$data[$i]['id'] = $post->ID;
		$data[$i]['title'] = $post->post_title;
        $data[$i]['slug'] = $post->post_name;
        $data[$i]['price'] = intval(get_field('price', $post->ID));
        $data[$i]['delivery'] = get_field('delivery', $post->ID);
		$i++;
	}

	return $data;
}

add_action('rest_api_init', function() {
    register_rest_route('wl/v1', 'products', [
		'methods' => 'GET',
		'callback' => 'wl_products',
	]);
});