github_quick_approve.user.js

// ==UserScript==
// @name     Github quick approve
// @version  5
// @grant    none
// @match    https://github.com/*
// @downloadURL https://gist.github.com/tomasv/68e6cb8940404ef0bd03b532766bb445/raw/github_quick_approve.user.js
// @updateURL https://gist.github.com/tomasv/68e6cb8940404ef0bd03b532766bb445/raw/github_quick_approve.user.js
// ==/UserScript==

let addButton = () => {
  if (document.getElementById('approve-button')) {
    return
  }

  let button = document.createElement('button')
  button.id = '#approve-button'
  button.className = 'diffbar-item btn btn-sm btn-primary'
  button.textContent = '👍🏻'
  button.style = 'margin-left: 5px'
  button.addEventListener('click', () => {
    let reviewScope = document.getElementById('submit-review')
    reviewScope.querySelector('[value=approve]').click()
    reviewScope.querySelector('form').submit()
  })
  document.getElementsByClassName('pr-review-tools')[0].appendChild(button)
}

window.addEventListener('DOMContentLoaded', addButton)
window.addEventListener('pjax:success', addButton)