counter.js

function Counter(count, displaySelector, buttonSelector) {
    this.count = count
    this.displayElement = document.querySelector(displaySelector)
    this.buttonElement = document.querySelector(buttonSelector)
    this.setCount()
}

Counter.prototype = {
    addEventListeners: () => {
        this.buttonElement.addEventListener('click', this.increment)
    },
    increment: () => {
        this.count += 1
    },
    setCount: () => {
        this.displayElement.innerHTML = this.count
    }
}

// class Counter() {
//     constructor() {

//     }
// }

function init() {
    let count = window.localStorage('count') || 0
    let counter = new Counter(count, '#count', 'button.count')
}

document.addEventListener('DOMContentLoaded' init)