gitLabAutoDelete.user.js

// ==UserScript==
// @name         GitLab - auto check 'delete source branch'
// @namespace    https://gist.github.com/LennyPenny
// @version      0.5
// @description  Auto checks the "delete source branch" option when creating merge requests
// @author       Lennart Bernhardt
// @updateURL    https://gist.github.com/LennyPenny/a0419284b7bea8f35620e238fefb3553/raw/gitLabAutoDelete.user.js
// @downloadURL  https://gist.github.com/LennyPenny/a0419284b7bea8f35620e238fefb3553/raw/gitLabAutoDelete.user.js
// @match        https://*/*/merge_requests/new*
// @require http://code.jquery.com/jquery-3.4.1.min.js
// @grant        GM_log
// ==/UserScript==

(function() {
    'use strict';
    var waitForEl = function(selector, callback) {
       if ($(selector).length) {
         callback();
        } else {
            setTimeout(function() {
                waitForEl(selector, callback);
            }, 100);
        }
    }

    function checkThem(nodes) {
        nodes.forEach(function(n) { n.checked = true })
    }

    waitForEl("#select2-chosen-2", function() {
        let sourceBranch = document.querySelector("#select2-chosen-2").textContent;
        if (sourceBranch == "master" || sourceBranch == "testing" || sourceBranch == "test" || sourceBranch == "production" || sourceBranch == "prod") {
            return;
        }

        let sel = "#merge_request_force_remove_source_branch";
        checkThem([].slice.call(document.querySelectorAll(sel)));
    });
})();