How to add Javascript in WordPress site

1.Simply Add this snippet in your functions.php file

function wpsnippetclub_javascript() {
    ?>
    <script>
        Your JavaScript
    </script>
    <?php
}
add_action('wp_head', 'wpsnippetclub_javascript');

2.Add the JS only in specific pages, this can help, because the code, doesn’t have to execute if not nececary.

function wpsnippetclub_javascript() { 
if (is_page ('42')) {  
    ?> 
    <script> 
        Your JavaScript 
    </script> 
    <?php 
   } 
} 
add_action('wp_head', 'wpsnippetclub_javascript');