SOURCE

let oBtn = document.querySelector("#btn")
        let oDiv = document.querySelector("div")
        oBtn.onclick = function(ev){
            var oEvent = ev || event;
            oDiv.style.display = "block"
            oEvent.cancelBubble = true
        }
        document.body.onclick = function(){
            oDiv.style.display = "none"
        }

<button id="btn">展示</button>
    <div class="menu"></div>

 *{margin: 0;padding: 0}
    body{
        padding: 50px;
    }
    .menu{
        width: 400px;
        height: 400px;
        background: #f00;
        display: none;
    }