code-export-syntax/chromium/assets/js/pages/export/html.js

const KEY_STORAGE_TNX = "tnx";

const SELECTOR = Object.freeze({
	COPY  : "#copy",
	HTML  : "#html",
	CLOSE : "#close"
});

class ExportToHtml {
	constructor() {
		this.#setContent();

		this.#bindClose();
		this.#bindCopyHtml();
	}

	/**
	 * Copy the contents of the textarea element onto the clipboard
	 * 
	 * @returns {Promise<boolean>}
	 */
	async copyHtml() {
		return await navigator.clipboard.writeText(document.querySelector(SELECTOR.HTML).value);
	}

	/**
	 * Set the textarea value with HTML from session storage
	 */
	async #setContent() {
		const content = await chrome.storage.session.get(KEY_STORAGE_TNX);

		document.querySelector(SELECTOR.HTML).value = content[KEY_STORAGE_TNX];
	}

	/**
	 * Bind button for copying textarea value
	 */
	#bindCopyHtml() {
		document.body.querySelector(SELECTOR.COPY).addEventListener("click", () => {
			this.copyHtml() ? alert("HTML Copied!") : alert("Failed to copy HTML");
		});
	}

	/**
	 * Bind button for closing this export window
	 */
	#bindClose() {
		document.querySelector(SELECTOR.CLOSE).addEventListener("click", () => window.close());
	}
}

globalThis._ExportToHtml = new ExportToHtml();