Fixed script to delete discord messages with new instructions provided because token is not present in localstorage anymore. Credits to "Niahoo" and "CarletonStuberg"

deletediscordmessages.js

// Credits to https://gist.github.com/niahoo/c99284a8908cd33d59b4aff802179e9b and https://gist.github.com/CarletonStuberg/0c838a6248772c6fea1339ddad503cce
// Turn on Developer Mode under User Settings > Appearance > Developer Mode (at the bottom)
// Then open the channel you wish to delete all of the messages (could be a DM) and click the three dots on the far right on your last message.
// Click "Copy ID" and paste that instead of LAST_MESSAGE_ID.

// Open the console with control+shift+i
// Go to network tab
// Move between two channels
// You should see a new request that says something along the lines "messages?limit=50"
// Open the request and find the header "authorization" and copy the RequestHeaders (you can save the authorization token for later).
// Go to the application tab in the console
// Go to http://discordapp.com under "Local Storage"
// Make sure a key with the token doesn't exist yet
// Create a new key with the name "token" and the value from the Authorization/Requestheaders.

// Copy / paste the below script into the JavaScript console.
// If you're in a DM you will receive a 403 error for every message the other user sent (you don't have permission to delete their messages).

var before = 'LAST_MESSAGE_ID';
clearMessages = function(){
	const authToken = document.body.appendChild(document.createElement`iframe`).contentWindow.localStorage.token.replace(/"/g, "");
	const channel = window.location.href.split('/').pop();
	const baseURL = `https://discordapp.com/api/channels/${channel}/messages`;
	const headers = {"Authorization": authToken };

	let clock = 0;
	let interval = 500;

	function delay(duration) {
		return new Promise((resolve, reject) => {
			setTimeout(() => resolve(), duration);
		});
	}

	fetch(baseURL + '?before=' + before + '&limit=100', {headers})
		.then(resp => resp.json())
		.then(messages => {
		return Promise.all(messages.map((message) => {
			before = message.id;
			return delay(clock += interval).then(() => fetch(`${baseURL}/${message.id}`, {headers, method: 'DELETE'}));
		}));
	}).then(() => clearMessages());
}
clearMessages();