SOURCE

const count = document.querySelector(".count");
const add = document.querySelector('.add');
const dec = document.querySelector('.dec');

var num = 0;
count.innerHTML = `${num}`;

add.addEventListener("click",function(){
    count.innerHTML = ``;
    num+=1;
    count.innerHTML = `${num}`;
});

dec.addEventListener("click",function(){
    count.innerHTML = ``;
    num-=1;
    count.innerHTML = `${num}`;
});

<div class="content">
	<h1 class="count"></h1>
	<div class="event">
		<button class="add">加一</button>
        <button class="dec">减一</button>
    </div>
</div>

.content{
    width: 100%;
    height: 500px;
    margin-top: 100px; 
}

.count{
    width: 100%;
    text-align: center;
    color: #93efed;
}

.event{
    display: flex;
    justify-content: center;
}

button{
    width: 75px;
    height: 50px;
    font-size: 25px;
    color: #44aec2;
    outline: none;
    border-radius: 20px;
    background: #362999;
}

button:hover{
    cursor: pointer;
}