js: Look up elements in the DOM as early as possible

Now that our script is loaded using <script defer>, we can rely on the
DOM's being complete as soon as it starts running.  So when we declare a
variable to point to a DOM element, we can initialise it with that
element.  This saves having these odd initialisations scattered around
the code, usually but not always at the site of first use.

I'd like to be able to do the same thing with the variables that point
to C functions, but the Module.cwrap() call isn't entirely safe before
Emscripten has finished loading the C code.

emccpre.js

@@ -14,6 +14,11 @@
  * couple of other helper functions.
  */
 
+// Because this script is run using <script defer>, we can guarantee
+// that the DOM is complete, so it's OK to look up elements
+// immediately.  On the other hand, the Emscripten runtime hasn't
+// started yet, so Module.cwrap isn't safe.
+
 // To avoid flicker while doing complicated drawing, we use two
 // canvases, the same size. One is actually on the web page, and the
 // other is off-screen. We do all our drawing on the off-screen one
@@ -102,7 +107,7 @@ var timer_reference;
 var timer_callback;
 
 // The status bar object, if we have one.
-var statusbar = null;
+var statusbar = document.getElementById("statusbar");
 
 // Currently live blitters. We keep an integer id for each one on the
 // JS side; the C side, which expects a blitter to look like a struct,
@@ -126,30 +131,32 @@ var dlg_return_sval, dlg_return_ival;
 
 // The <ul> object implementing the game-type drop-down, and a list of
 // the sub-lists inside it. Used by js_add_preset().
-var gametypelist = null;
+var gametypelist = document.getElementById("gametype");
-var gametypesubmenus = [];
+var gametypesubmenus = [gametypelist];
 
 // C entry point for miscellaneous events.
 var command;
 
 // The <form> encapsulating the menus.  Used by
 // js_get_selected_preset() and js_select_preset().
-var menuform = null;
+var menuform = document.getElementById("gamemenu");
 
 // The two anchors used to give permalinks to the current puzzle. Used
 // by js_update_permalinks().
-var permalink_seed, permalink_desc;
+var permalink_seed = document.getElementById("permalink-seed");
+var permalink_desc = document.getElementById("permalink-desc");
 
 // The undo and redo buttons. Used by js_enable_undo_redo().
-var undo_button, redo_button;
+var undo_button = document.getElementById("undo");
+var redo_button = document.getElementById("redo");
 
 // A div element enclosing both the puzzle and its status bar, used
 // for positioning the resize handle.
-var resizable_div;
+var resizable_div = document.getElementById("resizable");
 
 // Alternatively, an extrinsically sized div that we will size the
 // puzzle to fit.
-var containing_div;
+var containing_div = document.getElementById("puzzlecanvascontain");
 
 // Helper function to find the absolute position of a given DOM
 // element on a page, by iterating upwards through the DOM finding
@@ -371,12 +378,10 @@ function initPuzzle() {
         if (dlg_dimmer === null)
             command(6);
     };
-    undo_button = document.getElementById("undo");
     undo_button.onclick = function(event) {
         if (dlg_dimmer === null)
             command(7);
     };
-    redo_button = document.getElementById("redo");
     redo_button.onclick = function(event) {
         if (dlg_dimmer === null)
             command(8);
@@ -434,10 +439,6 @@ function initPuzzle() {
         }
     };
 
-    gametypelist = document.getElementById("gametype");
-    gametypesubmenus.push(gametypelist);
-    menuform = document.getElementById("gamemenu");
-
     // Find the next or previous item in a menu, or null if there
     // isn't one.  Skip list items that don't have a child (i.e.
     // separators) or whose child is disabled.
@@ -622,12 +623,6 @@ function initPuzzle() {
                                    ['number','number']);
     timer_callback = Module.cwrap('timer_callback', 'void', ['number']);
 
-    // Save references to the two permalinks and the status bar.
-    permalink_desc = document.getElementById("permalink-desc");
-    permalink_seed = document.getElementById("permalink-seed");
-    statusbar = document.getElementById("statusbar");
-
-    resizable_div = document.getElementById("resizable");
     if (resizable_div !== null) {
         var resize_handle = document.getElementById("resizehandle");
         var resize_xbase = null, resize_ybase = null, restore_pending = false;
@@ -697,7 +692,6 @@ function initPuzzle() {
      * on KaiOS 2.5, which doesn't have ResizeObserver.  Instead we
      * watch events that might indicate that the div has changed size.
      */
-    containing_div = document.getElementById("puzzlecanvascontain");
     if (containing_div !== null) {
         var resize_handler = function(event) {
             rescale_puzzle();