src/js/domevent.js
function memorizeHandler(element, type, keyFn, valueFn) {
    const map = safeEvent(element, type);
    let items = map.get(keyFn);

    if (items) {
        items.push(valueFn);
    } else {
        items = [valueFn];
        map.set(keyFn, items);
    }
}
function memorizeHandler(element, type, keyFn, valueFn) {
    const map = safeEvent(element, type);
    let items = map.get(keyFn);

    if (items) {
        items.push(valueFn);
    } else {
        items = [valueFn];
        map.set(keyFn, items);
    }
}
function memorizeHandler ( element , type , keyFn , valueFn ) { const map = safeEvent ( element , type ) ; let items = map . get ( keyFn ) ; if ( items ) { items . push ( valueFn ) ; } else { items = [ valueFn ] ; map . set ( keyFn , items ) ; } }
Memorize DOM event handler for unbinding
@param {HTMLElement} element - element to bind events
@param {string} type - events name
@param {function} keyFn - handler function that user passed at on() use
@param {function} valueFn - handler function that wrapped by domevent for
implementing some features
Memorize DOM event handler for unbinding