Check if Current Page Is a Parent or Child Page

WordPress has a built-in function to check if you are on a specific page:

if ( is_page(5) ) {
        // do some stuff
    }

Or whether the page is a child of a specific page:

if ( $post->post_parent == '5' ) {
        // do some stuff
    }

But there is no built-in WordPress function that checks both conditions together, and this is very useful. For example – if we want to load a JS file for a videos page and all pages under it…

The following function (add to functions.php) creates a new logical function for this check:

function is_tree($the_page_id) {   
    global $post;         

    if(is_page()&&($post->post_parent==$the_page_id||is_page($the_page_id))) 
       return true;   
    else 
       return false;   
    };

Usage

if (is_tree(5)) {
        // do some stuff
    }

For more on WordPress page templates, see WordPress Template Hierarchy.