Lets you delete every post from your tumblr pretty quickly, unless you have a *lot* of them

delete-tumblr-posts-100-at-a-time.js

/**
 * STEP 1: Go to https://www.tumblr.com/mega-editor/<your-blog-name>
 * STEP 2: Open your developer console
 * STEP 3: Paste this snippet in
 * STEP 4: Hit enter twice
 * STEP 5: Repeat from STEP 2 until everything is deleted
 *
 * Deletes 100 at a time due to tumblr restrictions.
 */
Array.from(document.querySelectorAll('.overlay')).slice(-100).forEach(el => el.click());
document.getElementById('delete_posts').click();