Add Javascript in header for a post, a page or a custom type post, add your code in a custom field with the name "js"

per_post_head_js.php

<?php
/**
* Add JavaScript code to the head section
*
* Add a custom field with the name "js" and this plugin will
* add it content to the head section of the page/post wrapped 
* correctly inside <script> tags.
*/
add_action('wp_head', 'per_post_head_js');
function per_post_head_js() {
	if ( $post = get_post(get_query_var('p')) ) {
		echo $js = get_post_meta( $post->ID, 'js', /* $single = */ true );
	}
}