getElementsByTagName

 
<!DOCTYPE HTML> 
<html> 
    <head> 
        <title>Example</title> 
    </head> 
    <body> 
        <pre id="results"></pre> 
        <p id="tblock"> 
            This is a HTML document. 
            <span id="banana">b</span> 
            <span id="apple">a</span>, 
            <span="orange">C</span="orange"> 
        </p> 
            <script> 
                
                var elems = document.getElementById("tblock")
                var spans = elems.getElementsByTagName("span"); 
                document.writeln(spans.length); 
            </script> 
    </body> 
</html>
Click to view the demo