“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

In a nutshell, if you're getting this error or similar error, that means only one thing. That is, in 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.

Now, 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:

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