Node.insertBefore()

Node.insertBefore() 方法將一個節點安插在參考節點之前，作為某個特定父節點之子節點。If the given child is a reference to an existing node in the document, insertBefore() moves it from its current position to the new position (there is no requirement to remove the node from its parent node before appending it to some other node).

同一個節點並不會同時出現在兩個地方。所以當節點已經有父節點，它會先被移除，然後被插入在新的位置上。The Node.cloneNode() can be used to make a copy of the node before appending it under the new parent. Note that the copies made with cloneNode will not be automatically kept in sync.

若參考節點為 null, 那需插入的節點就會成為特定父節點的最後一個子節點。

If the given child is a DocumentFragment, the entire contents of the DocumentFragment are moved into the child list of the specified parent node.

Syntax

var insertedNode = parentNode.insertBefore(newNode, referenceNode);

If referenceNode is null, the newNode is inserted at the end of the list of child nodes.

referenceNode 並非可選擇的參數 -- 一定要傳入一個節點或是 null。若是傳入失敗或不正確的參數，可能會依不同的瀏覽器版本而有不同的行為。

Returns

會回傳新加入的子節點。若該值(newNode)是DocumentFragment, 則回傳空的 DocumentFragment。

Example

<div id="parentElement">
   <span id="childElement">foo bar</span>
</div>

<script>
// Create the new node to insert
var newNode = document.createElement("span");

// Get a reference to the parent node
var parentDiv = document.getElementById("childElement").parentNode;

// Begin test case [ 1 ] : Exist a childElement --> All working correctly
var sp2 = document.getElementById("childElement");
parentDiv.insertBefore(newNode, sp2);
// End test case [ 1 ]

// Begin test case [ 2 ] : childElement is of Type undefined
var sp2 = undefined; // Not exist a node of id "childElement"
parentDiv.insertBefore(newNode, sp2); // Implicit dynamic cast to type Node
// End test case [ 2 ]

// Begin test case [ 3 ] : childElement is of Type "undefined" ( string )
var sp2 = "undefined"; // Not exist a node of id "childElement"
parentDiv.insertBefore(newNode, sp2); // Generate "Type Error: Invalid Argument"
// End test case [ 3 ]
</script>

insertedNode 被插入的節點，也就是 newNode。
parentNode 指定的父節點。
newNode 被插入的節點。
referenceNode 參考節點。The node before which newNode is inserted.

Example

<div id="parentElement">
  <span id="childElement">foo bar</span>
</div>

<script>
// Create a new, plain <span> element
var sp1 = document.createElement("span");

// Get a reference to the element, before we want to insert the element
var sp2 = document.getElementById("childElement");
// Get a reference to the parent element
var parentDiv = sp2.parentNode;

// Insert the new element into the DOM before sp2
parentDiv.insertBefore(sp1, sp2);
</script>

There is no insertAfter method. It can be emulated by combining the insertBefore method with nextSibling.

In the previous example, sp1 could be inserted after sp2 using:

parentDiv.insertBefore(sp1, sp2.nextSibling);

If sp2 does not have a next sibling, then it must be the last child — sp2.nextSibling returns null, and sp1 is inserted at the end of the child node list (immediately after sp2).

Example

Insert an element before the first child element, using the firstChild property.

// Get a reference to the element in which we want to insert a new node
var parentElement = document.getElementById('parentElement');
// Get a reference to the first child
var theFirstChild = parentElement.firstChild;

// Create a new element
var newElement = document.createElement("div");

// Insert the new element before the first child
parentElement.insertBefore(newElement, theFirstChild);

When the element does not have a first child, then firstChild is null. The element is still appended to the parent, after the last child. Since the parent element did not have a first child, it did not have a last child either. Consequently, the new element is the only element, after insertion.

Browser compatibility

BCD tables only load in the browser

Specifications

Specification	Status	Comment
DOM The definition of 'Node.insertBefore' in that specification.	Living Standard	Fixes errors in the insertion algorithm
DOM4 The definition of 'Node.insertBefore' in that specification.	Obsolete	Describes the algorithm in more detail
Document Object Model (DOM) Level 3 Core Specification The definition of 'Node.insertBefore' in that specification.	Obsolete	No notable changes
Document Object Model (DOM) Level 2 Core Specification The definition of 'Node.insertBefore' in that specification.	Obsolete	No notable changes
Document Object Model (DOM) Level 1 Specification The definition of 'Node.insertBefore' in that specification.	Obsolete	Introduced