GitHub: Set merge message to PR title and description

script.js

// ==UserScript==
// @name         GitHub: Set merge message to PR title and description
// @namespace    https://github.com/pencil
// @version      0.6
// @description  Set the default GitHub merge title and description to the one of the PR itself.
// @author       Nils Caspar
// @match        https://github.com/*
// @icon         https://github.githubassets.com/favicons/favicon.png
// @grant        none
// @downloadURL  https://gist.githubusercontent.com/pencil/51460c59d4c21e10e2d33f1a7208db04/raw/script.js
// @updateURL    https://gist.githubusercontent.com/pencil/51460c59d4c21e10e2d33f1a7208db04/raw/script.js
// @homepageURL  https://greasyfork.org/en/scripts/426735-github-set-merge-message-to-pr-title-and-description
// @supportURL   https://greasyfork.org/en/scripts/426735-github-set-merge-message-to-pr-title-and-description/feedback
// ==/UserScript==

(function() {
    'use strict';
    const $ = (str, el) => (el || document).querySelector(str),
          $$ = (str, el) => [...(el || document).querySelectorAll(str)];

    const githubApp = $('.application-main');
    githubApp.addEventListener('click', (event) => {
        const target = event.target;
        if(target.getAttribute('data-details-container') != '.js-merge-pr') {
            return;
        }

        console.info('updating merge message and title...');
        const mergeTitleField = $('#merge_title_field'),
              mergeDescriptionField = $('#merge_message_field'),
              prTitleContainer = $('.gh-header-title .js-issue-title'),
              prNumberContainer = $('.gh-header-title .color-fg-muted'),
              prDescriptionContainer = $('form.js-comment-update textarea');
        if(!prNumberContainer || !prNumberContainer.innerText) {
            console.info('could not find PR number');
            return;
        }
        if(!prTitleContainer || !prTitleContainer.innerText) {
            console.info('could not find PR title');
            return;
        }
        if(!mergeTitleField) {
            console.info('could not find merge title');
            return;
        }
        mergeTitleField.value = prTitleContainer.innerText.trim() + ' (' + prNumberContainer.innerText.trim() + ')';

        if(!prDescriptionContainer || !prDescriptionContainer.innerText) {
            console.info('could not find PR description');
            return;
        }
        if(!mergeDescriptionField) {
            console.info('could not find merge description');
            return;
        }
        mergeDescriptionField.value = prDescriptionContainer.value;
    });
})();