[#9655] Recover from malformed JSON checkout response

If a plugin or other code produces output during checkout processing,
it breaks the JSON parsing. This commit makes use of jQuery’s Ajax
dataFilter which allow us to sanitize the raw response before it’s
parsed by the Ajax handler.

assets/js/frontend/checkout.js

@@ -342,6 +342,36 @@ jQuery( function( $ ) {
 					});
 				}
 
+				jQuery.ajaxSetup( {
+					dataFilter: function( raw_response ) {
+
+						try {
+
+							// check for valid JSON
+							var data = $.parseJSON( raw_response );
+
+							if ( data && typeof data === 'object' ) {
+
+								// return it so it can be parsed by Ajax handler
+								return raw_response;
+							}
+
+						} catch ( e ) {
+
+							// attempt to fix the malformed JSON
+							valid_json = raw_response.match( /{"result.*"}/ );
+
+							if ( null === valid_json ) {
+								console.log( 'Unable to fix malformed JSON' );
+							} else {
+								raw_response = valid_json[0];
+							}
+						}
+
+						return raw_response;
+					}
+				} );
+
 				$.ajax({
 					type:		'POST',
 					url:		wc_checkout_params.checkout_url,