JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <a href="" id="one"></a>
    <span id="count">0</span>
    <a href="" id="two"></a>
</div>

CSS

div{
    position: relative;
    border: 1px solid green;
    width: 80px;
    margin: 100px auto;
    text-align: center;
    border-radius: 20px;
    overflow: hidden;
}
a{    
    position: relative;
    background-color: green;
    display: block;
    width: 20px;
    height: 20px;
    box-shadow: inset 0px 1px 1px white;
}
#one {
     float: left;
     border-radius: 10px 0px 0px 10px; 
}
#two {
     float: right;
     border-radius: 0px 10px 10px 0px;
}
#one:before {
    position: absolute;
    content: "";
    border-top: 7px solid transparent;
    border-bottom: 7px solid transparent;
    border-right: 7px solid white;
    left: 6px;
    top: 3px;
    }
#one:after {
    position: absolute;
    content: "";
    border-top: 7px solid transparent;
    border-bottom: 7px solid transparent;
    border-right: 7px solid green;
    left: 8px;
    top: 3px;
    }

#two:before {
    position: absolute;
    content: "";
    border-top: 7px solid transparent;
    border-bottom: 7px solid transparent;
    border-left: 7px solid white;
    right: 6px;
    top: 3px;
    }
#two:after {
    position: absolute;
    content: "";
    border-top: 7px solid transparent;
    border-bottom: 7px solid transparent;
    border-left: 7px solid green;
    right: 8px;
    top: 3px;
    }

JavaScript

var i = 0;
var count = document.getElementById("count");

document.getElementById("one").onclick = function() {
    i--;
    if (i < 0) {i = 0}; 
    count.innerHTML = i;
    return false; 
};

document.getElementById("two").onclick = function() {
    i++;
    count.innerHTML = i;
    return false;
};