Node.insertBefore() - Web APIs 编辑

The Node.insertBefore() method inserts a node before a reference node as a child of a specified parent node.

If the given node already exists in the document, insertBefore() moves it from its current position to the new position. (That is, it will automatically be removed from its existing parent before appending it to the specified new parent.)

This means that a node cannot be in two locations of the document simultaneously.

Note: 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.

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

let insertedNode = parentNode.insertBefore(newNode, referenceNode)

insertedNode

The node being inserted (the same as newNode)

parentNode

The parent of the newly inserted node.

newNode

The node to be inserted.

referenceNode

The node before which newNode is inserted. If this is null, then newNode is inserted at the end of parentNode's child nodes.

Note: referenceNode is not an optional parameter. You must explicitly pass a Node or null. Failing to provide it or passing invalid values may behave differently in different browser versions.

Return value

Returns the added child (unless newNode is a DocumentFragment, in which case the empty DocumentFragment is returned).

Example

Example 1

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

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

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

// Begin test case [ 1 ] : Existing childElement (all works correctly)
let sp2 = document.getElementById("childElement")
parentDiv.insertBefore(newNode, sp2)
// End test case [ 1 ]

// Begin test case [ 2 ] : childElement is of Type undefined
let sp2 = undefined // Non-existent 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 )
let sp2 = "undefined" // Non-existent node of id "childElement"
parentDiv.insertBefore(newNode, sp2) // Generates "Type Error: Invalid Argument"
// End test case [ 3 ]
</script>

Example 2

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

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

// Get the reference element
let sp2 = document.getElementById("childElement")
// Get the parent element
let parentDiv = sp2.parentNode

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

Note: There is no insertAfter() method. It can be emulated by combining the insertBefore method with Node.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 3

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

// Get the parent element
let parentElement = document.getElementById('parentElement')
// Get the parent's first child
let theFirstChild = parentElement.firstChild

// Create a new element
let 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 newly inserted element is the only element.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Node.removeChild()
• Node.replaceChild()
• Node.appendChild()
• Node.hasChildNodes()
• Element.insertAdjacentElement()
• ParentNode.prepend()
• ChildNode.before()
• ChildNode.after()