diff --git a/content/content.h b/content/content.h
index f8f8d32f1..c2605a7f1 100644
--- a/content/content.h
+++ b/content/content.h
@@ -72,7 +72,7 @@ typedef enum {
 	CONTENT_MSG_REFRESH,   /**< wants refresh */
 	CONTENT_MSG_DOWNLOAD,  /**< download, not for display */
 	CONTENT_MSG_LINK,      /**< RFC5988 link */
-	CONTENT_MSG_GETCTX,    /**< Javascript context */
+	CONTENT_MSG_GETTHREAD, /**< Javascript thread */
 	CONTENT_MSG_GETDIMS,   /**< Get viewport dimensions. */
 	CONTENT_MSG_SCROLL,    /**< Request to scroll content */
 	CONTENT_MSG_DRAGSAVE,  /**< Allow drag saving of content */
@@ -180,9 +180,9 @@ union content_msg_data {
 	struct content_rfc5988_link *rfc5988_link;
 
 	/**
-	 * CONTENT_MSG_GETCTX - Javascript context
+	 * CONTENT_MSG_GETTHREAD - Javascript context (thread)
 	 */
-	struct jscontext **jscontext;
+	struct jsthread **jsthread;
 
 	/**
 	 * CONTENT_MSG_GETDIMS - Get the viewport dimensions
diff --git a/content/handlers/html/html.c b/content/handlers/html/html.c
index d1b810fbc..26be58d73 100644
--- a/content/handlers/html/html.c
+++ b/content/handlers/html/html.c
@@ -757,8 +757,8 @@ void html_finish_conversion(html_content *htmlc)
 	 * object, but with its target set to the Document object (and
 	 * the currentTarget set to the Window object)
 	 */
-	if (htmlc->jscontext != NULL) {
-		js_fire_event(htmlc->jscontext, "load", htmlc->document, NULL);
+	if (htmlc->jsthread != NULL) {
+		js_fire_event(htmlc->jsthread, "load", htmlc->document, NULL);
 	}
 
 	/* convert dom tree to box tree */
@@ -896,20 +896,20 @@ dom_default_action_DOMNodeInserted_cb(struct dom_event *evt, void *pw)
 			}
 			if (htmlc->enable_scripting) {
 				/* ensure javascript context is available */
-				if (htmlc->jscontext == NULL) {
+				if (htmlc->jsthread == NULL) {
 					union content_msg_data msg_data;
 
-					msg_data.jscontext = &htmlc->jscontext;
+					msg_data.jsthread = &htmlc->jsthread;
 					content_broadcast(&htmlc->base,
-							CONTENT_MSG_GETCTX,
+							CONTENT_MSG_GETTHREAD,
 							&msg_data);
 					NSLOG(netsurf, INFO,
 					      "javascript context: %p (htmlc: %p)",
-					      htmlc->jscontext,
+					      htmlc->jsthread,
 					      htmlc);
 				}
-				if (htmlc->jscontext != NULL) {
-					js_handle_new_element(htmlc->jscontext,
+				if (htmlc->jsthread != NULL) {
+					js_handle_new_element(htmlc->jsthread,
 							(dom_element *) node);
 				}
 			}
@@ -1015,8 +1015,8 @@ dom_default_action_finished_cb(struct dom_event *evt, void *pw)
 {
 	html_content *htmlc = pw;
 
-	if (htmlc->jscontext != NULL)
-		js_event_cleanup(htmlc->jscontext, evt);
+	if (htmlc->jsthread != NULL)
+		js_event_cleanup(htmlc->jsthread, evt);
 }
 
 /* callback function selector
@@ -1136,7 +1136,7 @@ html_create_html_data(html_content *c, const http_parameter *params)
 	c->search_string = NULL;
 	c->scripts_count = 0;
 	c->scripts = NULL;
-	c->jscontext = NULL;
+	c->jsthread = NULL;
 
 	c->enable_scripting = nsoption_bool(enable_javascript);
 	c->base.active = 1; /* The html content itself is active */
diff --git a/content/handlers/html/html_internal.h b/content/handlers/html/html_internal.h
index 9b363dc8b..a64078143 100644
--- a/content/handlers/html/html_internal.h
+++ b/content/handlers/html/html_internal.h
@@ -151,8 +151,8 @@ typedef struct html_content {
 	unsigned int scripts_count;
 	/** Scripts */
 	struct html_script *scripts;
-	/** javascript context */
-	struct jscontext *jscontext;
+	/** javascript thread in use */
+	struct jsthread *jsthread;
 
 	/** Number of entries in stylesheet_content. */
 	unsigned int stylesheet_count;
diff --git a/content/handlers/html/html_object.c b/content/handlers/html/html_object.c
index 223f5516d..3a60c47f1 100644
--- a/content/handlers/html/html_object.c
+++ b/content/handlers/html/html_object.c
@@ -340,8 +340,9 @@ html_object_callback(hlcache_handle *object,
 		/* Don't care about favicons that aren't on top level content */
 		break;
 
-	case CONTENT_MSG_GETCTX:
-		*(event->data.jscontext) = NULL;
+	case CONTENT_MSG_GETTHREAD:
+		/* Objects don't have JS threads */
+		*(event->data.jsthread) = NULL;
 		break;
 
 	case CONTENT_MSG_GETDIMS:
diff --git a/content/handlers/html/html_script.c b/content/handlers/html/html_script.c
index 4df5f3384..301acadd6 100644
--- a/content/handlers/html/html_script.c
+++ b/content/handlers/html/html_script.c
@@ -42,7 +42,7 @@
 #include "html/html.h"
 #include "html/html_internal.h"
 
-typedef bool (script_handler_t)(struct jscontext *jscontext, const uint8_t *data, size_t size, const char *name);
+typedef bool (script_handler_t)(struct jsthread *jsthread, const uint8_t *data, size_t size, const char *name);
 
 
 static script_handler_t *select_script_handler(content_type ctype)
@@ -62,7 +62,7 @@ nserror html_script_exec(html_content *c, bool allow_defer)
 	script_handler_t *script_handler;
 	bool have_run_something = false;
 
-	if (c->jscontext == NULL) {
+	if (c->jsthread == NULL) {
 		return NSERROR_BAD_PARAMETER;
 	}
 
@@ -95,7 +95,7 @@ nserror html_script_exec(html_content *c, bool allow_defer)
 				size_t size;
 				data = content_get_source_data(
 						s->data.handle, &size );
-				script_handler(c->jscontext, data, size,
+				script_handler(c->jsthread, data, size,
 					       nsurl_access(hlcache_handle_get_url(s->data.handle)));
 				have_run_something = true;
 				/* We have to re-acquire this here since the
@@ -319,12 +319,12 @@ convert_script_sync_cb(hlcache_handle *script,
 
 		/* attempt to execute script */
 		script_handler = select_script_handler(content_get_type(s->data.handle));
-		if (script_handler != NULL && parent->jscontext != NULL) {
+		if (script_handler != NULL && parent->jsthread != NULL) {
 			/* script has a handler */
 			const uint8_t *data;
 			size_t size;
 			data = content_get_source_data(s->data.handle, &size );
-			script_handler(parent->jscontext, data, size,
+			script_handler(parent->jsthread, data, size,
 				       nsurl_access(hlcache_handle_get_url(s->data.handle)));
 		}
 
@@ -549,7 +549,7 @@ exec_inline_script(html_content *c, dom_node *node, dom_string *mimetype)
 	lwc_string_unref(lwcmimetype);
 
 	if (script_handler != NULL) {
-		script_handler(c->jscontext,
+		script_handler(c->jsthread,
 			       (const uint8_t *)dom_string_data(script),
 			       dom_string_byte_length(script),
 			       "?inline script?");
@@ -575,13 +575,13 @@ html_process_script(void *ctx, dom_node *node)
 	/* We should only ever be here if scripting was enabled for this
 	 * content so it's correct to make a javascript context if there
 	 * isn't one already. */
-	if (c->jscontext == NULL) {
+	if (c->jsthread == NULL) {
 		union content_msg_data msg_data;
 
-		msg_data.jscontext = &c->jscontext;
-		content_broadcast(&c->base, CONTENT_MSG_GETCTX, &msg_data);
-		NSLOG(netsurf, INFO, "javascript context %p ", c->jscontext);
-		if (c->jscontext == NULL) {
+		msg_data.jsthread = &c->jsthread;
+		content_broadcast(&c->base, CONTENT_MSG_GETTHREAD, &msg_data);
+		NSLOG(netsurf, INFO, "javascript context %p ", c->jsthread);
+		if (c->jsthread == NULL) {
 			/* no context and it could not be created, abort */
 			return DOM_HUBBUB_OK;
 		}
@@ -668,6 +668,6 @@ nserror html_script_free(html_content *html)
 /* exported internal interface documented in html/html_internal.h */
 nserror html_script_invalidate_ctx(html_content *htmlc)
 {
-	htmlc->jscontext = NULL;
+	htmlc->jsthread = NULL;
 	return NSERROR_OK;
 }
diff --git a/content/handlers/javascript/js.h b/content/handlers/javascript/js.h
index 522dd9879..126cb7cfb 100644
--- a/content/handlers/javascript/js.h
+++ b/content/handlers/javascript/js.h
@@ -25,15 +25,33 @@
 
 #include "utils/errors.h"
 
-typedef struct jscontext jscontext;
-typedef struct jsobject jsobject;
-
 struct dom_event;
 struct dom_document;
 struct dom_node;
 struct dom_element;
 struct dom_string;
 
+/**
+ * JavaScript interpreter heap
+ *
+ * In order to try and be moderately performant, we create a heap
+ * per browser window.  This heap is shared by all browsing contexts
+ * we end up creating in that window.
+ */
+typedef struct jsheap jsheap;
+
+/**
+ * JavaScript interpreter thread
+ *
+ * When we create a browsing context itself (window+content) we have
+ * to create a JS thread to attach to the browsing context.
+ *
+ * JS threads are associated with heaps and will be destroyed when
+ * the heap is destroyed.  They can be shut down manually though
+ * and should be for object lifetime safety reasons.
+ */
+typedef struct jsthread jsthread;
+
 /**
  * Initialise javascript interpreter
  */
@@ -45,41 +63,51 @@ void js_initialise(void);
 void js_finalise(void);
 
 /**
- * Create a new javascript context.
+ * Create a new javascript heap.
  *
- * There is usually one context per browsing context (browser window)
+ * There is usually one heap per browser window.
  *
  * \param timeout elapsed wallclock time (in seconds) before \a callback is called
- * \param jsctx Updated to the created JS context
+ * \param heap Updated to the created JS heap
  * \return NSERROR_OK on success, appropriate error otherwise.
  */
-nserror js_newcontext(int timeout, jscontext **jsctx);
+nserror js_newheap(int timeout, jsheap **heap);
 
 /**
- * Destroy a previously created context
+ * Destroy a previously created heap.
+ *
+ * \param heap The heap to destroy
  */
-void js_destroycontext(jscontext *ctx);
+void js_destroyheap(jsheap *heap);
 
 /**
- * Create a new javascript compartment
+ * Create a new javascript thread
  *
  * This is called once for a page with javascript script tags on
- * it. It constructs a fresh global window object.
+ * it. It constructs a fresh global window object and prepares the JS
+ * browsing context.  It's important that threads are shut down cleanly
+ * when the browsing context is going to be cleaned up.
+ *
+ * \param heap The heap to create the thread within
+ * \param win_priv The value to give to the Window constructor as the window
+ * \param doc_priv The value to give to the Document constructor as the document
+ * \param thread Updated to the created thread
+ * \return NSERROR_OK on success, appropriate error otherwise
  */
-jsobject *js_newcompartment(jscontext *ctx, void *win_priv, void *doc_priv);
+nserror js_newthread(jsheap *heap, void *win_priv, void *doc_priv, jsthread **thread);
 
 /**
  * execute some javascript in a context
  */
-bool js_exec(jscontext *ctx, const uint8_t *txt, size_t txtlen, const char *name);
+bool js_exec(jsthread *thread, const uint8_t *txt, size_t txtlen, const char *name);
 
 /**
  * fire an event at a dom node
  */
-bool js_fire_event(jscontext *ctx, const char *type, struct dom_document *doc, struct dom_node *target);
+bool js_fire_event(jsthread *thread, const char *type, struct dom_document *doc, struct dom_node *target);
 
 bool
-js_dom_event_add_listener(jscontext *ctx,
+js_dom_event_add_listener(jsthread *thread,
 			  struct dom_document *document,
 			  struct dom_node *node,
 			  struct dom_string *event_type_dom,
@@ -94,7 +122,7 @@ js_dom_event_add_listener(jscontext *ctx,
  * by the context provided.  The JS implementation must then scan the element
  * for on* attributes and register appropriate listeners for those handlers.
  */
-void js_handle_new_element(jscontext *ctx, struct dom_element *node);
+void js_handle_new_element(jsthread *thread, struct dom_element *node);
 
 /**
  * Handle an event propagation finished callback.
@@ -104,6 +132,6 @@ void js_handle_new_element(jscontext *ctx, struct dom_element *node);
  * it may need to perform before the DOM finishes and the event may end up
  * freed.
  */
-void js_event_cleanup(jscontext *ctx, struct dom_event *evt);
+void js_event_cleanup(jsthread *thread, struct dom_event *evt);
 
 #endif /* NETSURF_JAVASCRIPT_JS_H_ */
diff --git a/content/handlers/javascript/none/none.c b/content/handlers/javascript/none/none.c
index 26b9b5300..b79d3242f 100644
--- a/content/handlers/javascript/none/none.c
+++ b/content/handlers/javascript/none/none.c
@@ -35,35 +35,36 @@ void js_finalise(void)
 {
 }
 
-nserror js_newcontext(int timeout, jscontext **jsctx)
+nserror js_newheap(int timeout, jsheap **heap)
 {
-	*jsctx = NULL;
+	*heap = NULL;
 	return NSERROR_OK;
 }
 
-void js_destroycontext(jscontext *ctx)
+void js_destroyheap(jsheap *heap)
 {
 }
 
-jsobject *js_newcompartment(jscontext *ctx, void *win_priv, void *doc_priv)
+nserror js_newthread(jsheap *heap, void *win_priv, void *doc_priv, jsthread **thread)
 {
-	return NULL;
+	*thread = NULL;
+	return NSERROR_NOT_IMPLEMENTED;
 }
 
-bool js_exec(jscontext *ctx, const uint8_t *txt, size_t txtlen, const char *name)
+bool js_exec(jsthread *thread, const uint8_t *txt, size_t txtlen, const char *name)
 {
 	return true;
 }
 
-bool js_fire_event(jscontext *ctx, const char *type, struct dom_document *doc, struct dom_node *target)
+bool js_fire_event(jsthread *thread, const char *type, struct dom_document *doc, struct dom_node *target)
 {
 	return true;
 }
 
-void js_handle_new_element(jscontext *ctx, struct dom_element *node)
+void js_handle_new_element(jsthread *thread, struct dom_element *node)
 {
 }
 
-void js_event_cleanup(jscontext *ctx, struct dom_event *evt)
+void js_event_cleanup(jsthread *thread, struct dom_event *evt)
 {
 }
diff --git a/desktop/browser_private.h b/desktop/browser_private.h
index c9f9bbcf6..562abf131 100644
--- a/desktop/browser_private.h
+++ b/desktop/browser_private.h
@@ -263,7 +263,8 @@ struct browser_window {
 	bool can_edit;
 
 	/** current javascript context */
-	struct jscontext *jsctx;
+	struct jsheap *jsheap;
+	struct jsthread *jsthread;
 
 	/** cache of the currently displayed status text. */
 	struct {
diff --git a/desktop/browser_window.c b/desktop/browser_window.c
index 3d1a19f60..bd446a072 100644
--- a/desktop/browser_window.c
+++ b/desktop/browser_window.c
@@ -1485,15 +1485,19 @@ browser_window_callback(hlcache_handle *c, const hlcache_event *event, void *pw)
 		}
 		break;
 
-	case CONTENT_MSG_GETCTX:
-		/* only the content object created by the browser
-		 * window requires a new global compartment object
-		 */
-		assert(bw->loading_content == c);
-		if (js_newcompartment(bw->jsctx,
-				      bw,
-				      hlcache_handle_get_content(c)) != NULL) {
-			*(event->data.jscontext) = bw->jsctx;
+	case CONTENT_MSG_GETTHREAD:
+		{
+			/* only the content object created by the browser
+			 * window requires a new global compartment object
+			 */
+			jsthread *thread;
+			assert(bw->loading_content == c);
+			if (js_newthread(bw->jsheap,
+					 bw,
+					 hlcache_handle_get_content(c),
+					 &thread) == NSERROR_OK) {
+				*(event->data.jsthread) = thread;
+			}
 		}
 		break;
 
@@ -1760,8 +1764,8 @@ static void browser_window_destroy_internal(struct browser_window *bw)
 		bw->box = NULL;
 	}
 
-	if (bw->jsctx != NULL) {
-		js_destroycontext(bw->jsctx);
+	if (bw->jsheap != NULL) {
+		js_destroyheap(bw->jsheap);
 	}
 
 	/* These simply free memory, so are safe here */
@@ -3085,7 +3089,7 @@ browser_window_initialise_common(enum browser_window_create_flags flags,
 	assert(bw);
 
 	/* new javascript context for each window/(i)frame */
-	err = js_newcontext(nsoption_int(script_timeout), &bw->jsctx);
+	err = js_newheap(nsoption_int(script_timeout), &bw->jsheap);
 	if (err != NSERROR_OK)
 		return err;