Uncaught SyntaxError: Unexpected token u in JSON at position 0

Only at the checkout and on individual product pages I am getting the following error in the console log:

VM35594:1 Uncaught SyntaxError: Unexpected token u in JSON at position 0
    at JSON.parse (<anonymous>)
    at run (layout.min.js:9)
    at app.min.js:1
    at main.min.js:2
    at Object.execCb (require.min.js:112)
    at Module.check (require.min.js:56)
    at Module.<anonymous> (require.min.js:72)
    at require.min.js:11
    at require.min.js:74
    at each (require.min.js:3)

I am using a one page checkout extension, but when I disable that the error still shows. I thought it might had something to do with the reviews on the product page (as I moved the reviews out of the tabs), but undoing that change didn't fix the error on the product pages.

Try this in the console:

JSON.parse(undefined)

Here is what you will get:

Uncaught SyntaxError: Unexpected token u in JSON at position 0
    at JSON.parse (<anonymous>)
    at <anonymous>:1:6

In other words, your app is attempting to parse undefined, which is not valid JSON.

There are two common causes for this. The first is that you may be referencing a non-existent property (or even a non-existent variable if not in strict mode).

window.foobar = '{"some":"data"}';
JSON.parse(window.foobarn)  // oops, misspelled!

The second common cause is failure to receive the JSON in the first place, which could be caused by client side scripts that ignore errors and send a request when they shouldn't.

Make sure both your server-side and client-side scripts are running in strict mode and lint them using ESLint. This will give you pretty good confidence that there are no typos.

As @Seth Holladay @MinusFour commented, you are parsing an undefined variable.
Try adding an if condition before doing the parse.

if (typeof test1 !== 'undefined') { test2 = JSON.parse(test1); }

Note: This is just a check for undefined case. Any other parsing issues still need to be handled.

localStorage.clear()

That'll clear the stored data. Then refresh and things should start to work.

Your app is attempting to parse the undefined JSON web token. Such malfunction may occur due to the wrong usage of the local storage. Try to clear your local storage.

Example for Google Chrome:

1. F12
2. Application
3. Local Storage
4. Clear All

For me, that happened because I had an empty component in my page -

<script type="text/x-magento-init">
   {
   ".page.messages": {
       "Magento_Ui/js/core/app": []        
      }
   }

Deleting this piece of code resolved the issue.