“SyntaxError: Unexpected token < in JSON at position 0”

Question

In a React app component which handles Facebook-like content feeds, I am running into an error:

Feed.js:94 undefined \"parsererror\" \"SyntaxError: Un

Answer 1

On a general level this error occurs when a JSON object is parsed that has syntax errors in it. Think of something like this, where the message property contains unescaped double quotes:

{
    "data": [{
        "code": "1",
        "message": "This message has "unescaped" quotes, which is a JSON syntax error."
    }]
}

If you have JSON in your app somewhere then it's good to run it through JSONLint to verify that it doesn't have a syntax error. Usually this isn't the case though in my experience, it's usually JSON returned from an API that's the culprit.

When an XHR request is made to an HTTP API that returns a response with a Content-Type:application/json; charset=UTF-8 header which contains invalid JSON in the response body you'll see this error.

If a server-side API controller is improperly handling a syntax error, and it's being printed out as part of the response, that will break the structure of JSON returned. A good example of this would be an API response containing a PHP Warning or Notice in the response body:

Notice:  Undefined variable: something in /path/to/some-api-controller.php on line 99

{
    "success": false,
    "data": [{ ... }]
}

95% of the time this is the source of the issue for me, and though it's somewhat addressed here in the other responses I didn't feel it was clearly described. Hopefully this helps, if you're looking for a handy way to track down which API response contains a JSON syntax error I've written an Angular module for that.

Here's the module:

/**
 * Track Incomplete XHR Requests
 * 
 * Extend httpInterceptor to track XHR completions and keep a queue 
 * of our HTTP requests in order to find if any are incomplete or 
 * never finish, usually this is the source  of the issue if it's 
 * XHR related
 */
angular.module( "xhrErrorTracking", [
        'ng',
        'ngResource'
    ] )
    .factory( 'xhrErrorTracking', [ '$q', function( $q ) {
        var currentResponse = false;

        return {
            response: function( response ) {
                currentResponse = response;
                return response || $q.when( response );
            },
            responseError: function( rejection ) {
                var requestDesc = currentResponse.config.method + ' ' + currentResponse.config.url;
                if ( currentResponse.config.params ) requestDesc += ' ' + JSON.stringify( currentResponse.config.params );

                console.warn( 'JSON Errors Found in XHR Response: ' + requestDesc, currentResponse );

                return $q.reject( rejection );
            }
        };
    } ] )
    .config( [ '$httpProvider', function( $httpProvider ) {
        $httpProvider.interceptors.push( 'xhrErrorTracking' );
    } ] );

More details can be found in the blog article referenced above, I haven't posted everything found there here as it's probably not all relevant.