Error trying to render new page after fetch request in Express - node.js

Question

Problem
I have set up an HTTP fetch request from the client side with some JSON data (a dataURL from a canvas), and I want to be able to access it from the server side and then render a new page that displays that dataURL. This is an Express only app (no client-side frameworks).
However, I keep getting an error. It would seem to me that there's something wrong with the JSON data or the way I handle it, but look at the code.
In Safari, it's Unhandled Promise Rejection: SyntaxError: The string did not match the expected pattern.
In Chrome, it's Uncaught (in promise) SyntaxError: Unexpected token < in JSON at position 0
In Firefox, it's SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
Code
Client Side
I have an HTML canvas that I've converted into a dataURL and saved in a JSON object. That object gets sent to sendData.
If I get rid of the line response.json(), then I don't get the error. But I want to render a new page with the dataURL displayed in a paragraph tag.
function sendData(input) {
fetch('/', {
method: 'POST',
headers: {
'Content-Type': 'application/json'
},
body: JSON.stringify(input)
})
.then((response) => {
response.json()
})
}
//Saves dataURL as string
function saveData(input){
var dataURL = input.toDataURL()
data = { imageData: dataURL}
sendData(data)
}
Server Side
If I get rid of the line res.render('images'), I don't get the error, but, once again, I can't render the new page then. Also, this might be another issue, but Express is already not rendering the new page. Logging the req.body.imageData works.
exports.submit_image = function (req, res, next) {
console.log(req.body.imageData)
res.render('images')
}
Routes
And just in case this helps, here's the index.js in the routes directory
router.get('/', index.get_index)
router.post('/', index.submit_image)
How can I resolve this issue?

Answer 1

Node.js automatically handles rejected promises by logging them. You can suppress it using the following code. Take a look at this page for more information about promises.
window.addEventListener('unhandledrejection', event => {
event.preventDefault()
}, false)