SOURCE

var btn = document.getElementById("main-btn");

btn.addEventListener("mouseover", function(e) {
    var nav = document.getElementById("main-nav");
    var sub_btns = document.getElementsByClassName("sub-btn");
    var pos = [];
    e.className += "main-hover";
    console.log(e)
    nav.addEventListener("mouseover", function(e) {
        var total = 0;
        for (var x = 0; x < sub_btns.length; x++) {
          
                sub_btns[x].style.right = "-" + ((x + 1) * 30) + "%";
                pos[x] = ((x + 1) * 280);
            
            sub_btns[x].style.opacity = "1";
        }
        nav.style.width = 50 + "%";

    });

    nav.addEventListener("mouseout", function() {
        nav.style.width = "100px";

        for (var x = 0; x < sub_btns.length; x++) {
            sub_btns[x].style.left = "0";
            sub_btns[x].style.right = "0";
            sub_btns[x].style.opacity = "0";
        }
    })
});

<div class="wrapper">
    <div class="nav" id="main-nav">
        <div id="sub-navs">
            <button class="sub-btn"><a rel="nofollow" rel="noreferrer" href="#">link 1</a>
            </button>
            <button class="sub-btn"><a rel="nofollow" rel="noreferrer" href="#">link 2</a>
            </button>
            <button class="sub-btn"><a rel="nofollow" rel="noreferrer" href="#">link 3</a>
            </button>
            <button class="sub-btn"><a rel="nofollow" rel="noreferrer" href="#">link 4</a>
            </button>
        </div>
        <button id="main-btn">NAV</button>
    </div>
</div>

body,
html {
    background: #2e3941;
    height: 100%;
    width: 100%;
    overflow: hidden;
}
.wrapper {
    position: absolute;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
    margin: auto;
}
.nav {
    width: 100px;
    height: 38px;
    position: absolute;
    top: 0;
    z-index: 2;
    bottom: 0;
    left: 50%;
   
    -moz-transition: all 1s ease;
    -o-transition: all 1s ease;
    -webkit-transition: all 1s ease;
    transition: all 1s ease;
    transform: translate(-50%, -50%);
    top: 50%;
}
.nav button#main-btn {
    background: transparent;
    border: 0;
    padding: 10px;
    width: 100px;
    height: 50px;
    position: absolute;
    left: 0;
    right: 0;
    margin: auto;
    -moz-transition: all 0.5s ease;
    -o-transition: all 0.5s ease;
    -webkit-transition: all 0.5s ease;
    transition: all 0.5s ease;
    color: #fff;
    font-family: paytone one;
    top: 0;
    bottom: 0;
    -moz-border-radius: 10px;
    -webkit-border-radius: 10px;
    border-radius: 10px;
    font-size: 12px;
    transform-origin: center;
}
.nav button#main-btn:hover {
    -moz-transform: scale(1.5);
    -ms-transform: scale(1.5);
    -webkit-transform: scale(1.5);
    transform: scale(1.5);
}
.nav button#main-btn:focus {
    outline: none;
}
.nav #sub-navs button {
    background: transparent;
    width: 100px;
    height: 50px;
    border: 0;
    position: absolute;
    left: 0;
    right: 0;
    margin: auto;
    transition: all 1s ease, scale 0.5s ease, transform 0.5s ease;
    font-family: paytone one;
    top: 0;
    bottom: 0;
    opacity: 0;
    -moz-border-radius: 10px;
    -webkit-border-radius: 10px;
    border-radius: 10px;
    font-size: 12px;
    transform-origin: center;
    color: #2e3941 !important;
}
.nav #sub-navs button:hover {
    -moz-transform: scale(1.5);
    -ms-transform: scale(1.5);
    -webkit-transform: scale(1.5);
    transform: scale(1.5);
}
.nav #sub-navs button:focus {
    outline: none;
}
a {
    color: #fff;
    text-decoration: none;
}
.content {
    width: 65%;
    background: #fff;
    height: 100%;
    left: 0;
    right: 0;
    margin: auto;
}