ParentNode.firstElementChild 介面會會返回 ParentNode的第一個子元素(唯讀) ,如果該節點沒有子節點則返回null

This property was initially defined in the ElementTraversal pure interface. As this interface contained two distinct set of properties, one aimed at Node that have children, one at those that are children, they have been moved into two separate pure interfaces, ParentNode and ChildNode. In this case, firstElementChild moved to ParentNode. This is a fairly technical change that shouldn't affect compatibility.


var element = node.firstElementChild;


<ul id="foo">
  <li>First  (1)</li>
  <li>Second (2)</li>
  <li>Third  (3)</li>

var foo = document.getElementById('foo');
// yields: First  (1)

適用於IE8, IE9 和 Safari的Polyfill

// Overwrites native 'firstElementChild' prototype.
// Adds Document & DocumentFragment support for IE9 & Safari.
;(function(constructor) {
    if (constructor &&
        constructor.prototype &&
        constructor.prototype.firstElementChild == null) {
        Object.defineProperty(constructor.prototype, 'firstElementChild', {
            get: function() {
                var node, nodes = this.childNodes, i = 0;
                while (node = nodes[i++]) {
                    if (node.nodeType === 1) {
                        return node;
                return null;
})(window.Node || window.Element);


Specification Status Comment
The definition of 'ParentNode.firstElementChild' in that specification.
Living Standard Split the ElementTraversal interface in ChildNode and ParentNode. This method is now defined on the latter.
The Document and DocumentFragment implemented the new interfaces.
Element Traversal Specification
The definition of 'ElementTraversal.firstElementChild' in that specification.
Obsolete Added its initial definition to the ElementTraversal pure interface and use it on Element.


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