Basic javascript observable

Observable.js

function Observable(initialVal = undefined) {
    let staticValue = initialVal;
    let listeners = [];

    this.next = (val) => {
        staticValue = val;
        listeners.forEach(cb => cb(val));
    }
    this.subscribe = (listener) => {
        if(typeof listener === 'function') {
            listeners.push(listener);
        }        
    }
    this.unSubscribe = (listener) => {
        let listenerIdx = listeners.findIndex(cb => cb === listener);
        listeners.splice(listenerIdx, 1);
    }
}

Basic JavaScript observable. Create by calling the following code
const count = new Observable(0);