Unexpected token u in JSON at position 0 in JavaScript

December 8, 2022

Unexpected token u in JSON at position 0 in JavaScript

This guide is part of the “Common JavaScript Errors” series. It’s focused entirely on providing quick and easy solutions for JavaScript-related problems.


The "Unexpected token u in JSON at position 0" error in JavaScript is typically caused by trying to parse a string as JSON that is not valid JSON. This can happen if the string contains syntax errors or if the string is not actually JSON but is instead some other type of data, such as a JavaScript object.

Here is an example of how this error can occur:

const myString = '{ "key": "value" }';
const myObject = JSON.parse(myString);

In this example, the myString variable contains a string that is valid JSON. However, if the myString variable contained a string that was not valid JSON, such as a string with a syntax error, the JSON.parse() method would throw a "Unexpected token u in JSON at position 0" error.

To fix this error, you will need to ensure that the string you are trying to parse as JSON is actually valid JSON. This can typically be done by checking the string for syntax errors and correcting them, or by ensuring that the string is actually JSON and not some other type of data.

Here is an example of how you can fix this error by checking the string for syntax errors and correcting them:

const myString = '{ "key": "value" }';

try {
  const myObject = JSON.parse(myString);
} catch (error) {
  if (error instanceof SyntaxError) {
    console.error('Invalid JSON:', error.message);
  } else {
    throw error;
  }
}

In this example, the try/catch statement is used to catch any errors thrown by the JSON.parse() method. If the error is a SyntaxError, it is assumed to be caused by an invalid JSON string, and a message is printed to the console. Otherwise, the error is re-thrown.

Other common reasons for this error include:

  • Syntax errors: JSON is a strict format, and any syntax errors in the string will cause the JSON.parse() method to throw an error. For example, if the string is missing a closing quotation mark or if it contains an extra comma.
  • Unicode escape: Additionally, this error can occur if the string contains Unicode escape sequences that are not valid JSON. Unicode escape sequences represent Unicode characters in JSON strings, but an error is thrown if the escape sequence is invalid.

Local storage

When using local storage to store JSON values, it is important to ensure that the values you are storing are actually JSON. If you try to store a non-JSON value in local storage, you may encounter errors when retrieving the value from local storage and parsing it as JSON.

To avoid this problem, you can use the JSON.stringify() method to convert a non-JSON value to JSON before storing it in local storage.

const myObject = {
  key: 'value',
};

// Convert the object to JSON
const myJSON = JSON.stringify(myObject);

// Store the JSON in local storage
localStorage.setItem('myJSON', myJSON);

And here is an example of doing that for an Array:

const myArray = [1, 2, 3];

// Convert the array to JSON
const myJSON = JSON.stringify(myArray);

// Store the JSON in local storage
localStorage.setItem('myJSON', myJSON);