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

Posted by: admin November 30, 2017 Leave a comment

Questions:

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

Feed.js:94 undefined “parsererror” “SyntaxError: Unexpected token < in JSON at position 0

I ran into a similar error which turned out to be a typo in the HTML within the render function, but that doesn’t seem to be the case here.

More confusingly, I rolled the code back to an earlier, known-working version and I’m still getting the error.

Feed.js:

import React from 'react';

var ThreadForm = React.createClass({
  getInitialState: function () {
    return {author: '', 
            text: '', 
            included: '',
            victim: ''
            }
  },
  handleAuthorChange: function (e) {
    this.setState({author: e.target.value})
  },
  handleTextChange: function (e) {
    this.setState({text: e.target.value})
  },
  handleIncludedChange: function (e) {
    this.setState({included: e.target.value})
  },
  handleVictimChange: function (e) {
    this.setState({victim: e.target.value})
  },
  handleSubmit: function (e) {
    e.preventDefault()
    var author = this.state.author.trim()
    var text = this.state.text.trim()
    var included = this.state.included.trim()
    var victim = this.state.victim.trim()
    if (!text || !author || !included || !victim) {
      return
    }
    this.props.onThreadSubmit({author: author, 
                                text: text, 
                                included: included,
                                victim: victim
                              })
    this.setState({author: '', 
                  text: '', 
                  included: '',
                  victim: ''
                  })
  },
  render: function () {
    return (
    <form className="threadForm" onSubmit={this.handleSubmit}>
      <input
        type="text"
        placeholder="Your name"
        value={this.state.author}
        onChange={this.handleAuthorChange} />
      <input
        type="text"
        placeholder="Say something..."
        value={this.state.text}
        onChange={this.handleTextChange} />
      <input
        type="text"
        placeholder="Name your victim"
        value={this.state.victim}
        onChange={this.handleVictimChange} />
      <input
        type="text"
        placeholder="Who can see?"
        value={this.state.included}
        onChange={this.handleIncludedChange} />
      <input type="submit" value="Post" />
    </form>
    )
  }
})

var ThreadsBox = React.createClass({
  loadThreadsFromServer: function () {
    $.ajax({
      url: this.props.url,
      dataType: 'json',
      cache: false,
      success: function (data) {
        this.setState({data: data})
      }.bind(this),
      error: function (xhr, status, err) {
        console.error(this.props.url, status, err.toString())
      }.bind(this)
    })
  },
  handleThreadSubmit: function (thread) {
    var threads = this.state.data
    var newThreads = threads.concat([thread])
    this.setState({data: newThreads})
    $.ajax({
      url: this.props.url,
      dataType: 'json',
      type: 'POST',
      data: thread,
      success: function (data) {
        this.setState({data: data})
      }.bind(this),
      error: function (xhr, status, err) {
        this.setState({data: threads})
        console.error(this.props.url, status, err.toString())
      }.bind(this)
    })
  },
  getInitialState: function () {
    return {data: []}
  },
  componentDidMount: function () {
    this.loadThreadsFromServer()
    setInterval(this.loadThreadsFromServer, this.props.pollInterval)
  },
  render: function () {
    return (
    <div className="threadsBox">
      <h1>Feed</h1>
      <div>
        <ThreadForm onThreadSubmit={this.handleThreadSubmit} />
      </div>
    </div>
    )
  }
})

module.exports = ThreadsBox

In Chrome developer tools, the error seems to be coming from this function:

 loadThreadsFromServer: function loadThreadsFromServer() {
    $.ajax({
      url: this.props.url,
      dataType: 'json',
      cache: false,
      success: function (data) {
        this.setState({ data: data });
      }.bind(this),
      error: function (xhr, status, err) {
        console.error(this.props.url, status, err.toString());
      }.bind(this)
    });
  },

with the line console.error(this.props.url, status, err.toString() underlined.

Since it looks like the error seems to have something to do with pulling JSON data from the server, I tried starting from a blank db, but the error persists. The error seems to be called in an infinite loop presumably as React continuously tries to connect to the server and eventually crashes the browser.

EDIT:

I’ve checked the server response with Chrome dev tools and Chrome REST client, and the data appears to be proper JSON.

EDIT 2:

It appears that though the intended API endpoint is indeed returning the correct JSON data and format, React is polling http://localhost:3000/?_=1463499798727 instead of the expected http://localhost:3001/api/threads.

I am running a webpack hot-reload server on port 3000 with the express app running on port 3001 to return the backend data. What’s frustrating here is that this was working correctly the last time I worked on it and can’t find what I could have possibly changed to break it.

Answers:

The wording of the error message corresponds to what you get from Google Chrome when you run JSON.parse('<...'). I know you said the server is setting Content-Type:application/json, but I am led to believe the response body is actually HTML.

Feed.js:94 undefined "parsererror" "SyntaxError: Unexpected token < in JSON at position 0"

with the line console.error(this.props.url, status, err.toString()) underlined.

The err was actually thrown within jQuery, and passed to you as a variable err. The reason that line is underlined is simply because that is where you are logging it.

I would suggest that you add to your logging. Looking at the actual xhr (XMLHttpRequest) properties to learn more about the response. Try adding console.warn(xhr.responseText) and you will most likely see the HTML that is being received.

Questions:
Answers:

You’re receiving HTML (or XML) back from the server, but the dataType: json is telling jQuery to parse as JSON. Check the “Network” tab in Chrome dev tools to see contents of the server’s response.

Questions:
Answers:

This ended up being a permissions problem for me. I was trying to access a url I didn’t have authorization for with cancan, so the url was switched to users/sign_in. the redirected url responds to html, not json. The first character in a html response is <.

Questions:
Answers:

I my case the error was a result of me not assigning my return value to a variable. The following caused the error message:

return new JavaScriptSerializer().Serialize("hello");

I changed it to:

string H = "hello";
return new JavaScriptSerializer().Serialize(H);

Without the variable JSON is unable to properly format the data.

Questions:
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In my case, I was getting this running webpack, and it turned out to be some corruption somewhere in the local node_modules dir.

rm -rf node_modules
npm install

…was enough to get it working right again.

Questions:
Answers:

I experienced this error “SyntaxError: Unexpected token m in JSON at position”, where the token ‘m’ can be any other characters.

It turned out that I missed one of the double quotes in the JSON object when I was using RESTconsole for DB test, as {“name: “math”}, the correct one should be {“name”: “math”}

It took me a lot effort to figure out this clumsy mistake. I am afraid others would run into similar bummers.

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My problem was that I was getting the data back in a string which was not in a proper JSON format, which I was then trying to parse it. simple example: JSON.parse('{hello there}') will give an error at h. In my case the callback url was returning an unnecessary character before the objects: employee_names([{"name":.... and was getting error at e at 0. My callback URL itself had an issue which when fixed, returned only objects.

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In my case (for an Azure hosted Angular 2/4 site, my API call to mySite/api/… was redirecting due to mySite routing issues. So, it was returning the HTML from the redirected page instead of the api JSON. I added an exclusion in a web.config file for the api path.

I was not getting this error when developing locally because the Site and API were on different ports. There is probably a better way to do this … but it worked.

<?xml version="1.0" encoding="UTF-8"?>

<configuration>
    <system.webServer>
        <rewrite>
        <rules>
        <clear />

        <!-- ignore static files -->
        <rule name="AngularJS Conditions" stopProcessing="true">
        <match url="(app/.*|css/.*|fonts/.*|assets/.*|images/.*|js/.*|api/.*)" />
        <conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
        <action type="None" />
        </rule>

        <!--remaining all other url's point to index.html file -->
        <rule name="AngularJS Wildcard" enabled="true">
        <match url="(.*)" />
        <conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
        <action type="Rewrite" url="index.html" />
        </rule>

        </rules>
        </rewrite>
    </system.webServer>
</configuration>

Questions:
Answers:

I had the same error message following a tutorial. Our issue seems to be ‘url: this.props.url’ in the ajax call. In React.DOM when you are creating your element, mine looks like this.

ReactDOM.render(
    <CommentBox data="/api/comments" pollInterval={2000}/>,
    document.getElementById('content')
);

Well, this CommentBox does not have a url in its props, just data. When I switched url: this.props.url -> url: this.props.data, it made the right call to the server and I got back the expected data.

I hope it helps.

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After spending a lot of time with this, I found out that in my case the problem was having “homepage” defined on my package.json file made my app not work on firebase (same ‘token’ error).
I created my react app using create-react-app, then I used the firebase guide on the READ.me file to deploy to github pages, realized I had to do extra work for the router to work, and switched to firebase. github guide had added the homepage key on package.json and caused the deploy issue.

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Protip: Testing json on a local Node.js server? Make sure you don’t already have something routing to that path

'/:url(app|assets|stuff|etc)';

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Just to add to the answers It also happens when your API response includes


<?php{username: 'Some'}

which could be a case when your backend is using PHP.

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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:

<b>Notice</b>:  Undefined variable: something in <b>/path/to/some-api-controller.php</b> on line <b>99</b><br />
{
    "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.

Questions:
Answers:

This error occurs when you define the response as application/json and you are getting a HTML as a response. Basically, this happened when you are writing server side script for specific url with a response of JSON but the error format is in HTML.

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For me, this happened when one of the properties on the object I was returning as JSON threw an exception.

public Dictionary<string, int> Clients { get; set; }
public int CRCount
{
    get
    {
        var count = 0;
        //throws when Clients is null
        foreach (var c in Clients) {
            count += c.Value;
        }
        return count;
    }
}

Adding a null check, fixed it for me:

public Dictionary<string, int> Clients { get; set; }
public int CRCount
{
    get
    {
        var count = 0;
        if (Clients != null) {
            foreach (var c in Clients) {
                count += c.Value;
            }
        }
        return count;
    }
}

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just something basic to check, make sure you dont have anything commented out in the json file

//comments here will not be parsed and throw error

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Unexpected token < in JSON at position 0

A simple solution to this error is to write a comment in styles.less file.

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