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

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.

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.

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.

SyntaxError: Unexpected token < in JSON at position 0

You are getting an HTML file (or XML) instead of json.

Html files begin with <!DOCTYPE html>.

I “achieved” this error by forgetting the https:// in my fetch method:

fetch(`/api.github.com/users/${login}`)
    .then(response => response.json())
    .then(setData);

I verified my hunch:

I logged the response as text instead of JSON.

fetch(`/api.github.com/users/${login}`)
    .then(response => response.text())
    .then(text => console.log(text))
    .then(setData);

Yep, an html file.

Solution:

I fixed the error by adding back the https:// in my fetch method.

fetch(`https://api.github.com/users/${login}`)
    .then(response => response.json())
    .then(setData)
    .catch(error => (console.log(error)));

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 an HTML response is <.

In my case, I was getting this running webpack. It turned out to be corrupted somewhere in the local node_modules dir.

rm -rf node_modules
npm install

…was enough to get it working right again.

In a nutshell, if you’re getting this error or a similar error, that means only one thing: Someplace in our codebase, we were expecting a valid JSON format to process, and we didn’t get one. For example,

var string = "some string";
JSON.parse(string)

will throw an error, saying

Uncaught SyntaxError: Unexpected token s in JSON at position 0

Because, the first character in string is s & it’s not a valid JSON now. This can throw error in between also. like:

var invalidJSON= '{"foo" : "bar", "missedquotehere : "value" }';
JSON.parse(invalidJSON)

Will throw error:

VM598:1 Uncaught SyntaxError: Unexpected token v in JSON at position 36

because we intentionally missed a quote in the JSON string invalidJSON at position 36.

And if you fix that:

var validJSON= '{"foo" : "bar", "missedquotehere" : "value" }';
JSON.parse(validJSON)

will give you an object in JSON.

This error can be thrown in any place & in any framework/library. Most of the time you may be reading a network response which is not valid JSON. So steps of debugging this issue can be like:

curl or hit the actual API you’re calling.
Log/Copy the response and try to parse it with JSON.parse. If you’re getting error, fix it.
If not, make sure your code is not mutating/changing the original response.

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 will run into similar issues.

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

I was facing the same issue.

I removed the dataType:'json' from the $.ajax method.

Those who are using create-react-app and trying to fetch local json files.

As in create-react-app, webpack-dev-server is used to handle the request and for every request it serves the index.html. So you are getting

SyntaxError: Unexpected token < in JSON at position 0.

To solve this, you need to eject the app and modify the webpack-dev-server configuration file.

You can follow the steps from here.

For future googlers:

This message will be generated if the server-side function crashes.

Or if the server-side function doesn’t even exist ( i.e. Typo in function name ).

So – suppose you are using a GET request… and everything looks perfect and you’ve triple-checked everything…

Check that GET string one more time. Mine was:

'/theRouteIWant&someVar=Some value to send'

should be

'/theRouteIWant?someVar=Some value to send'
               ^

CrAsH ! ( … invisibly, on the server …)

Node/Express sends back the incredibly (non)helpful message:
Uncaught (in promise) SyntaxError: Unexpected token < in JSON at position 0.

Per Eliezer Berlin’s comment, the < token is usually a piece of unexpected HTML, such as in <head>, being returned by the server.

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>

2022 UPDATE: having written this several years ago. I’d call this suggestion more of a workaround – a direct fix. The better hosting pattern is to simply not try to host these api paths under your website path; rather, host them on separate base URLs entirely. For my use case example, the API and Web path would be entirely separate Azure Web Services and would get different URL endpoints.

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.

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.

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.

I got the same error while calling an API in React using the fetch API with the POST method.

Before:

fetch('/api/v1/tour',{
      method:"POST",
      headers:{"Content-type":"json/application"},
      body:JSON.stringify(info)
    })
    .then((response)=>response.json())
    .then((json)=>{
      if(json.status === 'success')
        alert(json.message)
      else
        console.log('something went wrong :(')
    }).catch(e=>console.log(e))

I resolved the error by changing the headers to {"Content-type":"application/json"}:

After:

fetch('/api/v1/tour',{
      method:"POST",
      headers:{"Content-type":"application/json"},
      body:JSON.stringify(info)
    })
    .then((response)=>response.json())
    .then((json)=>{
      if(json.status === 'success')
        alert(json.message)
      else
        console.log('something went wrong :(')
    }).catch(e=>console.log(e))

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.

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)';

In python you can use json.Dump(str) before send result to html template.
with this command string convert to correct json format and send to html template. After send this result to JSON.parse(result) , this is correct response and you can use this.

Make sure that response is in JSON format otherwise fires this error.

The possibilities for this error are overwhelming.

In my case, I found that the issue was adding the homepage filed in package.json caused the issue.

Worth checking: in package.json change:

homepage: "www.example.com"

hompage: ""

Malformed JSON or HTML instead of JSON is the underlying cause of this issue, as described by the other answers, however in my case I couldn’t reliably replicate this error, as if the server was sometimes returning valid JSON, and other times returning something else like an HTML error page or similar.

In order to avoid it breaking the page altogether, I resorted to manually trying to parse the returned content, and share it in case it helps anyone else resolve it for them.

const url = "https://my.server.com/getData";

fetch(url).then(response => {
  if (!response.ok) return; // call failed

  response.text().then(shouldBeJson => { // get the text-only of the response
    let json = null;
    try {
      json = JSON.parse(shouldBeJson); // try to parse that text
    } catch (e) {
      console.warn(e); // json parsing failed
      return;
    };
    if (!json) return; // extra check just to make sure we have something now.

    // do something with my json object
  });
});

While this obviously doesn’t resolve the root cause of the issue, it can still help to handle the issue a bit more gracefully and take some kind of reasonable action in instances when it fails.

For the React app made by CRA there are two main problems we might face while fetching the JSON data of any <dummy.json>
file.

I have my dummy.json file in my project and am trying to fetch the JSON data from that file but I got two errors:

"SyntaxError: Unexpected token < in JSON at position 0 .

I got an HTML file rather than actual JSON Data in the response in the Network tab in Chrome or any browser.

Here are the main two reasons behind that which solved my issue.

Your JSON data is invalid in your JSON file.
It might be that the JSON file did not load properly for this so you just restart your React server. This is my issue, within React.

React direct running or access the public folder not the src folder.

How I solved it:

I moved my file into the public folder and access is directly in any file of the src folder.

Making a REST call in the Redux action.js:

export const fetchDummy = ()=>{
return (dispatch)=>{
        dispatch(fetchDummyRequest());
        fetch('./assets/DummyData.json')
        .then(response => {
            if (!response.ok) {
                throw new Error("HTTP error " + response.status);
            }
            return response.json();
        })
        .then(result => {
            dispatch(fetchDummySuccess(result))
        })
        .catch(function (err) {
          dispatch(fetchDummyFailure(err))
        })
    }
}

In most cases, I get this error, when API throws an error with text format.
What I suggest is to see the error text content to figure out what is wrong going on.

jQuery =>
set dataType: ‘text’
fetch =>
reponse.text() instead of reponse.json()

This error usually occurs when HTML is returned by the endpoint.

So, check the endpoint address that you are making the request.
If it’s misspelled or if there are some unhandled invalid values in the request, these may be the reason for the endpoint returning HTML response.

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.

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;
    }
}

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

For some, this may help you guys:
I had a similar experience with WordPress REST API. I even used Postman to check if I had the correct routes or endpoint. I later found out that I accidentally put an “echo” inside my script – hooks:

Debug & check your console

Cause of the error

So basically, this means that I printed a value that isn’t JSON that is mixed with the script that causes AJAX error – “SyntaxError: Unexpected token r in JSON at position 0”

In my case (backend), I was using res.send(token);

Everything got fixed when I changed to res.send(data);

You may want to check this if everything is working and posting as intended, but the error keeps popping up in your front-end.

Filed under: Kiến thức lập trình - @ 06:34

Thẻ: javascriptnode.jsreactjsjsonrest

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Danh mục

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

You are getting an HTML file (or XML) instead of json.

I verified my hunch:

Solution: