Kotlin Firefox Extension popup.kt part 2 https://medium.com/@Cypressious/your-second-firefox-extension-in-kotlin-bafd91d87c41

popup.pt2.kt

fun listenForClicks() {
    document.addEventListener("click", { e ->
        val target = e.target as? Element ?: return@addEventListener

        browser.tabs.query(Query(active = true, currentWindow = true))
                .then({ tabs -> handleClick(target, tabs[0].id) })
                .catch(::reportError)
    })
}

fun handleClick(target: Element, id: Int) {
    if (target.classList.contains("beast")) {
        val url = getUrl(target.textContent)

        browser.tabs.insertCSS(id, CssDetails(CSS_HIDE_PAGE))
        browser.tabs.sendMessage(id, jsObject {
            command = "beastify"
            beastURL = url
        })
    } else {
        browser.tabs.removeCSS(id, CssDetails(CSS_HIDE_PAGE))
        browser.tabs.sendMessage(id, jsObject {
            command = "reset"
        })
    }
}

fun getUrl(name: String?): String {
    val relative = "beasts/${name?.toLowerCase()}.jpg"
    return browser.extension.getURL(relative)
}

const val CSS_HIDE_PAGE = """
    body > :not(.beastify-image) {
        display: none;
    }
"""

inline fun jsObject(init: dynamic.() -> Unit): dynamic {
    val o = js("{}")
    init(o)
    return o
}