javascript - Why does parent have to be written in front of the insertBefore function, but it is not needed when applied to other functions?

Question

function insertAfter(newElement,targetElment) {
    var parent = targetElment.parentNode;
    if(parent.lastChild == targetElment) {
        parent.appendChild(newElement); 
    } else {
        parent.insertBefore(newElement,targetElment.nextSibling);
    }
}

Answer 1

Because the DOM Node does not have this method, if you want to use it this way, just implant the method into the Node prototype

Node.prototype.insertAfter = function insertAfter(newNode, targetNode) {
  if (!(this instanceof Node)) {
    throw new TypeError('Illegal invocation')
  }
  
  if (arguments.length < 2) {
    throw new TypeError("Failed to execute 'insertAfter' on 'Node': 2 arguments required, but only " + arguments.length +" present.")
  }
  
  if (!(newNode instanceof Node)) {
    throw new TypeError("Failed to execute 'insertAfter' on 'Node': parameter 1 is not of type 'Node'.")
  }
  
  if (targetNode !== null && !(targetNode instanceof Node)) {
    throw new TypeError("Failed to execute 'insertAfter' on 'Node': parameter 2 is not of type 'Node'.")
  }
  
  if (targetNode !== null && targetNode.parentNode !== this) {
    throw new DOMException("Failed to execute 'insertAfter' on 'Node': The node before which the new node is to be inserted is not a child of this node.")
  }

  if(!targetNode || this.lastChild == targetNode) {
    this.appendChild(newNode); 
  } else {
    this.insertBefore(newNode, targetNode.nextSibling);
  }
}

This way you can use some processing of parent.insertAfter(newElement, targetElment)，这里模仿了 insertBefore.