JSON return null

By use AJAX, JSON return null

Problem: When use AJAX and when transferring array using json_encode() function json_decode() return null.

Solution: This situation may be because of not correct data presentation before call json_encode(). For example, numbers that has type string instead int. Check it can using var_dump(). In my case was so:

array(2) { [0]=> string(3) "212" [1]=> string(3) "211" }

and must be so:

array(2) { [0]=> int(212) [1]=> int(211) }

In my case, that situation has arisen because of this code:

<?php
$args = [
	'post_type' => 'custom_post_type',
	'posts_per_page' => -1,
	'fields' => 'ids',
	'no_found_rows' => true,
];
	
$custom_post_type = new WP_Query( $args );

if ( $custom_post_type->have_posts() ) {
	$custom_post_type_ids = [];
	while ( $custom_post_type->have_posts() ) {
		$custom_post_type->the_post();
		$custom_post_type_ids[] .= $post;
	}
	wp_reset_postdata();
}

Yeah, sure we could've add (int) function to $testimonials_ids[] .= (int)$post; that is by converting string to integer, but better do this way: 

Step 1. Change argument query 'fields' => 'ids' to 'fields' => 'id=>parent'

Step 2. Use function wp_list_pluck() exactly for such an occasion:

<?php
if ( $custom_post_type->have_posts() ) {
	$custom_post_type_ids = wp_list_pluck( $custom_post_type->posts, 'ID' );
}

thus, ultimately our code would be so:

<?php
$args = [
	'post_type' => 'custom_post_type',
	'posts_per_page' => -1,
	'fields' => 'id=>parent',
	'no_found_rows' => true,
];
	
$custom_post_type = new WP_Query( $args );

if ( $custom_post_type->have_posts() ) {
	$custom_post_type_ids = wp_list_pluck( $custom_post_type->posts, 'ID' );
}