Delete all messages in a Discord channel

delete-all-messages.js

// 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.
// Copy/paste the below script into the JavaScript console.
// invoke the function `clearMessages("username", "messageId")`
// where username is optional username to filter off of and where messageID is an optional ID that you wish to retain, but delete everything before

function clearMessages(username, before) {
  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 };

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

  function clearMessages(before) {
    let clock = 0;
    let interval = 500;
    let beforeQuery = before == null ? '' : `&before=${before}`;
    fetch(`${baseURL}?limit=100${beforeQuery}`, {headers})
      .then(resp => resp.json())
      .then(messages => {
        before = messages[messages.length - 1].id;        
        return username == null 
          ? messages 
          : messages.filter(msg => msg.author.username === username);
      }).then(messages => Promise.all(messages.map(msg => {
          return delay(clock += interval)
            .then(() => fetch(`${baseURL}/${msg.id}`, {headers, method: 'DELETE'}));
      })))
      .then(() => clearMessages(before));
  }

  clearMessages(before);
}