on.js

var stop = eventTarget.on("event", function optionalFilter(event) {
    return event.target.classList.contains("some-class")
}, function listener(event) {
    // handle event
})

// stop listening()
stop()

// sugar

function matchesSelector(selector) {
    return function (event) {
        return event.target.matchesSelector(selector)
    }
}

eventTarget.on("event", matchesSelector(".some-class"), listener)