How to add javascript code in header for 1 page only!!

<?php if (is_page('about')) {
   ==   javascript code here ==
<?php } ?>

sub your page slug for the word about

Add this to your theme’s functions.php file:

if ( is_page( 'about' ) {
	wp_enqueue_script( 'unique-id', get_stylesheet_directory_uri() . '/name-of-file.js', false, 0.1 );
}

Just change the name and/or directory of the file. You might also want to change unique-id.

Sources:
https://codex.www.ads-software.com/Function_Reference/wp_enqueue_script

If you use greenshady’s code above there is a ) missing in the first line:
if ( is_page( 'about' )) {

Enjoy !