Introduction to using replaceChild() to replace existing elements with javascript_javascript skills

var oDiv = document.getElementById("guoDiv");
var oSpan = document.createElement("span ");
oSpan.innerHTML = "4";
var firsChild = oDiv.firstElementChild ? oDiv.firstElementChild : oDiv.firstChild
var returnNode = oDiv.replaceChild(oSpan, firsChild); //Replace the first element An element, returns the replaced element
alert(returnNode.innerHTML); //1
var lastChild = oDiv.lastElementChild ? oDiv.lastElementChild : oDiv.lastChild;
oSpan = document.createElement( "span");
oSpan.innerHTML = "5";
returnNode = oDiv.replaceChild(oSpan, lastChild); //Replace the last one and return the replaced element
alert(returnNode.innerHTML );//3

< ;div id="guoDiv">
1
2
3
< /div>