irccloud upload to imgur userscript

// ==UserScript==
// @name         Upload to custom services
// @namespace    http://github.com/hnOsmium0001/
// @version      1.0
// @description  Add option to upload to custom image hosting services
// @author       You
// @match        https://*.irccloud.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function hasClassPrefix(element, prefix) {
        if (!element.getAttribute) return false;

        const classes = (element.getAttribute("class") || "").split();
        return classes.some(x => x.startsWith(prefix));
    }

    class ChildrenSelector {
        constructor(elm) {
            this.elm = elm;
        }

        andThenTag(tag, alternativeElm) {
            if (!this.elm) {
                this.elm = alternativeElm;
                return this;
            }

            for (const child of this.elm.childNodes) {
                if (child.tagName === tag) {
                    this.elm = child;
                    return this;
                }
            }
            this.elm = alternativeElm;
            return this;
        }

        andThenClass(prefix, alternativeElm) {
            if (!this.elm) {
                this.elm = alternativeElm;
                return this;
            }

            for (const child of this.elm.childNodes) {
                if (hasClassPrefix(child, prefix)) {
                    this.elm = child;
                    return this;
                }
            }
            // Failed to find a matching children
            this.elm = alternativeElm;
            return this;
        }

        accept(successful, failed) {
            if (this.elm) {
                successful(this.elm);
            } else {
                failed();
            }
        }
    }

    const uploadContainer = document.getElementById('fileUploadForm');
    let batchFile;
    new ChildrenSelector(uploadContainer)
        .andThenClass("batch")
        // TODO
        .accept(e => { batchFile = e }, () => {});
    let singleFile;
    new ChildrenSelector(uploadContainer)
        .andThenClass("single")
        .andThenClass("previewWrapper")
        .accept(e => { singleFile = e }, () => {});

    let inputBox;
    (function checkSession() {
        function init() {
            // cb() is a global function provided by irccloud
            inputBox = document.getElementById("bufferInputView" + cb().bid())
        }

        if (window.hasOwnProperty('SESSION')) {
            window.SESSION.bind('init', function () {
                init();
                window.SESSION.buffers.on('doneSelected', function () {
                    init();
                });
            });
        } else {
            setTimeout(checkSession, 100);
        }
    })();

    function uploadImage(imageData) {
        const FILTER = ";base64,";
        const data = new FormData();
        data.append("image", imageData.substring(imageData.indexOf(FILTER) + FILTER.length));

        const req = new XMLHttpRequest();
        req.onreadystatechange = () => {
            if (req.readyState == 4) {
                // Response should contain a URL to the uploaded image
                const response = JSON.parse(req.response);
                inputBox.value += response.data.link;
            }
        };
        // For whatever reason, thea ction endpoint for uploading image is /image instead of /upload as described at https://apidocs.imgur.com/#c85c9dfc-7487-4de2-9ecd-66f727cf3139
        // See https://stackoverflow.com/questions/55733271/upload-image-to-imgur-failed-because-of-cors
        req.open("POST", "https://api.imgur.com/3/image");
        req.setRequestHeader("Authorization", "Client-ID YOUR_CLIENT_ID");
        req.send(data);
    }

    new ChildrenSelector(uploadContainer)
        .andThenClass("buttons")
        .accept(uploadButtonContainer => {
            const uploadBtn = document.createElement("button");
            const closeBtn = uploadButtonContainer.lastElementChild;
            uploadButtonContainer.prepend(uploadBtn);
            uploadBtn.type = "button";
            uploadBtn.classList.add("action");
            uploadBtn.classList.add("confirm");
            uploadBtn.innerHTML = "<span>Upload to Imgur</span>";
            uploadBtn.addEventListener("click", () => {
                console.log("Uploading to imgur");
                if (singleFile.hasChildNodes()) {
                    const img = singleFile.children[0].children[0];
                    uploadImage(img.src);
                    closeBtn.click();
                }
                if (batchFile.hasChildNodes()) {
                    closeBtn.click();
                }
            });
        }, () => console.log("Failed to find button list of fileUploadContainer"));
})();