js-on-sx: parseFloat recognises Infinity prefix

lib/js/runtime.sx

@@ -4505,11 +4505,33 @@
   js-parse-float-prefix
   (fn
     (s)
-    (let
-      ((end (js-float-prefix-end s 0 false false false)))
-      (cond
-        ((= end 0) (js-nan-value))
-        (else (js-parse-num-safe (js-string-slice s 0 end)))))))
+    (cond
+      ((js-float-has-infinity-prefix? s 0)
+        (js-infinity-value))
+      ((and
+         (>= (len s) 1)
+         (= (char-at s 0) "+")
+         (js-float-has-infinity-prefix? s 1))
+        (js-infinity-value))
+      ((and
+         (>= (len s) 1)
+         (= (char-at s 0) "-")
+         (js-float-has-infinity-prefix? s 1))
+        (- 0 (js-infinity-value)))
+      (else
+        (let
+          ((end (js-float-prefix-end s 0 false false false)))
+          (cond
+            ((= end 0) (js-nan-value))
+            (else (js-parse-num-safe (js-string-slice s 0 end)))))))))
+
+(define
+  js-float-has-infinity-prefix?
+  (fn
+    (s i)
+    (and
+      (>= (len s) (+ i 8))
+      (= (js-string-slice s i (+ i 8)) "Infinity"))))
 
 (define
   js-float-prefix-end