Update js/small-menu.js

Rather than using the number `600` in multiple places it makes more sense to create a variable that holds the value. It is easier to read and it is easier to update.

js/small-menu.js

@@ -2,6 +2,7 @@
  * Handles toggling the main navigation menu for small screens.
  */
 jQuery( document ).ready( function( $ ) {
+	var smallScreenWidth = 600;
 	var $masthead = $( '#masthead' ),
 	    timeout = false;
 
@@ -16,7 +17,7 @@ jQuery( document ).ready( function( $ ) {
 	};
 
 	// Check viewport width on first load.
-	if ( $( window ).width() < 600 )
+	if ( $( window ).width() < smallScreenWidth )
 		$.fn.smallMenu();
 
 	// Check viewport width when user resizes the browser window.
@@ -27,7 +28,7 @@ jQuery( document ).ready( function( $ ) {
 			clearTimeout( timeout );
 
 		timeout = setTimeout( function() {
-			if ( browserWidth < 600 ) {
+			if ( browserWidth < smallScreenWidth ) {
 				$.fn.smallMenu();
 			} else {
 				$masthead.find( '.site-navigation' ).removeClass( 'main-small-navigation' ).addClass( 'main-navigation' );
@@ -36,4 +37,4 @@ jQuery( document ).ready( function( $ ) {
 			}
 		}, 200 );
 	} );
-} );
+} );