SOURCE

let _subscribe = function () {
    class Sub {
        constructor() {
            this.$pond = [];
        }
        add(func) {
            let flag = this.$pond.some(item => {
                return func === item;
            })

            !flag ? this.$pond.push(func) : null;
        }

        remove(func) {
            let $pond = this.$pond;
            for (let i = 0; i < $pond.length; i++) {
                if ($pond[i] === func) {
                    this.$pond[i] === null;
                    break;
                }
            }
        }

        fire(...args) {
            let $pond = this.$pond;
            for (let i = 0; i < $pond.length; i++) {
                if ($pond[i] === null) {
                    this.$pond.splice(i, 1)
                    continue;
                }
                $pond[i].call(this, ...args);
            }
        }
    }

    return function subscribe() {
        return new Sub();
    }();
}

let pond = _subscribe();
document.getElementById('submit').onclick = function (ev) {
    console.log(pond);
    pond.fire(ev);
}

let fn1 = function () {
    console.log(1);
}

let fn2 = function () {
    console.log(2);
    pond.remove(fn1);
}

let fn3 = function () {
    console.log(3);
}

let fn4 = function (ev) {
    console.log(4,ev);
}

pond.add(fn1);
pond.add(fn2);
pond.add(fn3);
pond.add(fn4);

<div>
    <button id='submit'>excute</button>
</div>