webextensions-examples/context-menu-copy-link-with-types/background.js

// Callback reads runtime.lastError to prevent an unchecked error from being 
// logged when the extension attempt to register the already-registered menu 
// again. Menu registrations in event pages persist across extension restarts.
browser.contextMenus.create({
        id: "copy-link-to-clipboard",
        title: "Copy link to clipboard",
        contexts: ["link"],
    },
    // See https://extensionworkshop.com/documentation/develop/manifest-v3-migration-guide/#event-pages-and-backward-compatibility
    // for information on the purpose of this error capture.
    () => void browser.runtime.lastError,
);

browser.contextMenus.onClicked.addListener((info, tab) => {
    if (info.menuItemId === "copy-link-to-clipboard") {
        // Examples: text and HTML to be copied.
        const text = "This is text: " + info.linkUrl;
        // Always HTML-escape external input to avoid XSS.
        const safeUrl = escapeHTML(info.linkUrl);
        const html = `This is HTML: <a href="${safeUrl}">${safeUrl}</a>`;

        // The example will show how data can be copied, but since background
        // pages cannot directly write to the clipboard, we will run a content
        // script that copies the actual content.

        // clipboard-helper.js defines function copyToClipboard.
        const code = "copyToClipboard(" +
            JSON.stringify(text) + "," +
            JSON.stringify(html) + ");";

        browser.tabs.executeScript({
            code: "typeof copyToClipboard === 'function';",
        }).then((results) => {
            // The content script's last expression will be true if the function
            // has been defined. If this is not the case, then we need to run
            // clipboard-helper.js to define function copyToClipboard.
            if (!results || results[0] !== true) {
                return browser.tabs.executeScript(tab.id, {
                    file: "clipboard-helper.js",
                });
            }
        }).then(() => {
            return browser.tabs.executeScript(tab.id, {
                code,
            });
        }).catch((error) => {
            // This could happen if the extension is not allowed to run code in
            // the page, for example if the tab is a privileged page.
            console.error("Failed to copy text: " + error);
        });
    }
});

// https://gist.github.com/Rob--W/ec23b9d6db9e56b7e4563f1544e0d546
function escapeHTML(str) {
    // Note: string cast using String; may throw if `str` is non-serializable, e.g. a Symbol.
    // Most often this is not the case though.
    return String(str)
        .replace(/&/g, "&amp;")
        .replace(/"/g, "&quot;").replace(/'/g, "&#39;")
        .replace(/</g, "&lt;").replace(/>/g, "&gt;");
}