Add initState parameter

README.md

@@ -20,6 +20,12 @@ If one of the sections (`above`/`below`) contains HTML (e.g. from another shortc
       unsafe = true
 ```
 
+# Parameters
+
+| Name           | Accepted values     | Default      | Description |
+| ----           | ---                 | ---          | ---         |
+| `initState`    | `closed`, `open`    | `closed`     | The initial state of the collapsible |
+
 # Demo
 
 ```

layouts/collapse.html

@@ -1,4 +1,10 @@
-<div class="hugo-collapse-parent">
+<!-- parameter sanity check -->
+
+{{ if not (in "open closed" (default "closed" (.Get "initState"))) }}
+  {{ errorf "Failed to handle page %q: param \"initState\" only takes [open, closed]; got %s" .Page.Path (.Get "init")}}
+{{ end }}
+
+<div class="hugo-collapse-parent {{ if eq (default "closed" (.Get "initState")) "closed" }}closed{{ end }}">
   <!-- prevent common resources from being loaded more than once per page -->
   {{ if not ($.Page.Scratch.Get "collapseLoaded") }}
     {{ $.Page.Scratch.Set "collapseLoaded" 1 }}

static/js/collapse.js

@@ -12,7 +12,7 @@ var callback = function() {
     a.addEventListener('click', toggleCollapse, false);
     a.className = 'collapseAnchor';
     a.href="#"; /* without this it's not keyboard focusable */
-    wrap.classList.add('closed', 'collapseWrap');
+    wrap.classList.add('collapseWrap');
   } // for hook
   function toggleCollapse(e) {
     e.preventDefault();