callback.js

// once 参数表示是否只是触发一次
const wrapCallback = (fn, once = false) => ({ callback: fn, once });

EventEmitter.prototype.addListener = function(type, fn, once = false) {
    let handler = this.events.get(type);
    if (!handler) {
        // 为 type 事件绑定回调
        this.events.set(type, wrapCallback(fn, once));
    } else if (handler && typeof handler.callback === "function") {
        // 目前 type 事件只有一个回调
        this.events.set(type, [ handler, wrapCallback(fn, once) ]);
    } else {
        // 目前 type 事件回调数 >= 2
        handler.push(wrapCallback(fn, once));
    }
};

EventEmitter.prototype.removeListener = function(type, listener) {
    let handler = this.events.get(type);
    if (!handler) return;
    if (!Array.isArray(handler)) {
        if (handler.callback === listener.callback) this.events.delete(type);
        else return;
    }
    for (let i = 0; i < handler.length; i++) {
        let item = handler[i];
        if (item.callback === listener.callback) {
            // 删除该回调，注意数组塌陷的问题，即后面的元素会往前挪一位。i 要 --
            handler.splice(i, 1);
            i--;
            if (handler.length === 1) {
                // 长度为 1 就不用数组存了
                this.events.set(type, handler[0]);
            }
        }
    }
};

EventEmitter.prototype.once = function(type, fn) {
    this.addListener(type, fn, true);
};
EventEmitter.prototype.emit = function(type, ...ang) {};