为什么不能通过document.getElementById（“ node's_id”）。child选择子节点？

Question

Suppose I have a simple html code sonsisiting a parent div and child span element as:

<div id="my_div"><span> Nothin' but the G thang </span></div>

It is said that every element in html DOM is an object. So we can select child objects as parent.child . Eg window.document , window.document.body , window.document.head etc then my question is why does document.getElementById("my_div").span return undefined?

Similarly window.document.body works but I can't go one level down after that with window.document.body.div .

Answer 1

With . ( dot-notation ), you are trying to access the span property of the object ( Node/Element ) returned by document.getElementById . As there is no span property associated with returned object, document.getElementById("my_div").span will be undefined

Use document.querySelector , Returns the first element within the document (using depth-first pre-order traversal of the document's nodes|by first element in document markup and iterating through sequential nodes by order of amount of child nodes) that matches the specified group of selectors

 console.log(document.querySelector('#my_div span')); 

 <div id="my_div"><span> Nothin' but the G thang </span> </div> 

Or Node.chilren , read-only property that returns a live HTMLCollection of the child elements of Node.

 console.log(document.getElementById('my_div').children[0]); 

 <div id="my_div"><span> Nothin' but the G thang </span> </div> 

Answer 2

You can select span like this

 var span = document.getElementById("my_div").querySelector('span'); console.log(span); 

 <div id="my_div"><span> Nothin' but the G thang </span> </div> 

Or in your case you can also use firstChild

 var span = document.getElementById("my_div").firstChild; console.log(span); 

 <div id="my_div"><span>Nothin' but the G thang </span></div>