@ndn/util/lib/event_browser.js

NDNts: general utilities

import { getOrInsert } from "./iter_browser.js";
/**
 * Keep records on whether an event listener has been added.
 * @param target - EventTarget to override.
 * @returns Map from event type to whether listeners may exist.
 *
 * @remarks
 * This may allow `EventTarget` subclass to skip certain event generation code paths.
 * Tracking is imprecise: it does not consider `options.once` and `options.signal`.
 */
export function trackEventListener(target) {
    const m = new Map();
    const { addEventListener, removeEventListener } = target;
    Object.defineProperties(target, {
        addEventListener: {
            configurable: true,
            value(...args) {
                const [evt, fn] = args;
                const record = getOrInsert(m, evt, () => [0, new WeakSet()]);
                if (!record[1].has(fn)) {
                    record[1].add(fn);
                    ++record[0];
                }
                addEventListener.call(this, ...args);
            },
        },
        removeEventListener: {
            configurable: true,
            value(...args) {
                removeEventListener.call(this, ...args);
                const [evt, fn] = args;
                const record = m.get(evt);
                if (!record) {
                    return;
                }
                if (record[1].delete(fn)) {
                    --record[0];
                }
                if (record[0] === 0) {
                    m.delete(evt);
                }
            },
        },
    });
    return new Proxy({}, {
        get(target, prop) {
            void target;
            return m.has(prop);
        },
    });
}