WordPress admin-ajax.php JSON parser - Parse JSON string that may contain HTML elements before the output. See https://katz.co/?p=7671

maybe-parse-json.js

/**
 * Parse JSON string that may contain HTML elements - requires jQuery
 *
 * @param {string} string JSON text to attempt to parse
 * @returns {object} Either JSON-parsed object or object with a message key containing an error message
 */
function maybe_parse_json( string ) {
	var json_object;

	// Parse valid JSON
	try {
		json_object = jQuery.parseJSON( string );
	} catch ( exception ) {

		// The JSON didn't parse most likely because PHP warnings.
		// We attempt to strip out all content up to the expected JSON `{"`
		var second_try = string.replace( /((.|\n)+?){"/gm, "{\"" );

		try {
			json_object = $.parseJSON( second_try );
		} catch ( exception ) {

			// If it doesn't work the second time, just log the error
			console.log( '*** \n*** \n*** Error-causing response:\n***\n***\n', string );

			// And return an error message.
			json_object = {
				message: 'JSON failed: another plugin caused a conflict with completing this request. Check your browser\'s Javascript console to view the invalid content.'
			};
		}
	}

	return json_object;
}