appendchild element once if element is not present in javascript code example

Example 1: add element to body javascript

var elem = document.createElement('div');
elem.style.cssText = 'position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000';
document.body.appendChild(elem);

Example 2: for each append to document

var person = {
    firstName: "john",
    lastName: "doe",
    age: 45,
    placeOfBirth: "somewhere"
}

for(var key in person) {
    
        var p = document.createElement("p");
        p.innerHTML = person[key];
        document.body.appendChild(p)
    
}

Example 3: appendchild element once if element presense in js

<script>
       var button = false;
        function  myfun(){
            if ( button === false){
                var b1 = document.createElement("BUTTON");
                b1.innerHTML = "Click Me";
                document.body.appendChild(b1); 
                return button = true;  
            }
      }
    </script>