`Unexpected token < in JSON` error when passing PHP array to JSON - wordpress-rest-api

Question

If anybody can point me in the right direction on this one, I will be eternally grateful!
I’m playing around with the WordPress register_rest_field() function (for the first time) to add a custom field to an existing endpoint in the Rest API.
Here is the snippet where I’ve registered my field:
register_rest_field(
'country',
'featured_post',
array(
'get_callback' => 'dd_return_featured_sound'
)
);
And here is the callback function:
function dd_return_featured_sound( $object, $field_name, $request ) {
$posts = get_field( 'featured_sound', $object['taxonomy'] . '_' . $object['id'] );
$post = $posts[0]; // $posts is an array, but there is only 1 featured post
return array(
'link' => get_the_permalink( $post ),
'title' => get_the_title( $post ),
'terms' => get_the_terms( $post, $object['taxonomy'] ),
'image' => get_the_post_thumbnail( $post )
);
}
The problem is that I keep getting the following error in the console:
Uncaught SyntaxError: Unexpected token < in JSON at position 0
Here’s the relevant snippet of the js:
if (request.status >= 200 && request.status < 400) {
const data = JSON.parse(request.responseText);
// if there are terms to show
if (data.length) {
data.forEach(term => {
console.log(term);
});
}
}
I’ve been trying to work this one out for the a couple of hours now and I’m close to tearing my hair out (I have a healthy head of hair and would very much like to keep it that way).
If anybody can help at all, like I said before, I'd be eternally grateful!

Answer 1

Solved. Literally banging my head against the desk in disbelief at the fact that I didn’t realise this right away…
It transpires that the featured_sound field is in fact not mandatory, so naturally everything broke in the callback function when trying to get [0] of a non-existent array.